Is it necessary to have a clause after word “that”

Question

It has long been known that the sea otters living along the West Coast of North America help keep kelp forests in their habitat healthy and vital

A. NO CHANGE

B. living along the West Coast of North America, they help

C. that live along the West Coast of North America and help to

D. that live along the West Coast of North America, where they help

Here, B can be eliminated because it creates a comma splice. D can be eliminated because it converts an important information in the sentence as non-essential.

Now, when it comes to picking between A nd C, A I thought could be eliminated because it doesn't encloses the non-essential information in a comma pair.

C seems to be okay. However, the correct answer is A. Does it means that all the sentences which come after that need to be a complete clause with a subject and a verb? As in the given question, "and help" have no subject to refer to; maybe it refers to "that," acting as the subject.

Please explain me this question.

Answer 1

"That" can be a pronoun, determiner, adverb, or conjunction. When it's used as a conjunction, it does need a complete clause. In its other meanings, it doesn't. For instance "Is that hot?" is perfectly fine.

Now, when it comes to picking between A nd C, A I thought could be eliminated because it doesn't encloses the non-essential information in a comma pair.

I don't think "essential" is the best term. The question is whether it is restrictive. The phrase "living along the West Coast of North America" is restrictive; it restricts the subject from sea otters in general to a subset of sea otters. Restrictive clauses should not be set off with commas.

Does it means that all the sentences which come after that need to be a complete clause with a subject and a verb?

When you're talking about a word, rather than using the word, it should be set off from the other words somehow, e.g.

Does it means that all the sentences which come after "that" need to be a complete clause with a subject and a verb?

I don't know of a way to do sentence diagrams in SE, so I'll just use parentheses:

It has long been known (that the sea otters (that live along the West Coast of North America, (where they help keep kelp forests in their habitat healthy and vital)))

Here, "where they help keep kelp forests in their habitat healthy and vital" is a subordinate clause to "the West Coast of North America".

"(that live along the West Coast of North America, (where they help keep kelp forests in their habitat healthy and vital)))" is a restrictive clause modifying "sea otters". So the entire passage "the sea otters (that live along the West Coast of North America, (where they help keep kelp forests in their habitat healthy and vital)))" is a noun phrase; it introduces the phrase "sea otters", then further describes what sea otters they're talking about. The important part is that In Option $D$, everything that follows "sea otters" is modifying "sea otters". While there are verbs in that description, they are in subclauses, and none of them have "sea otters" as a subject. So for the purpose the analysis, we can replace "the sea otters ..." with "certain sea otters".

Now the sentence becomes "It has long been known that certain sea otters". That's the entire sentence. Remember, everything else in the sentence is just saying which sea otters they're talking about. This then is a sentence fragment; it doesn't present a complete thought that can be the object of the verb "known".

Answer 2

While many English classes seem to teach the use of commas as immutable grammar rules, this is misleading. Commas are almost always optional, and are used to separate parts of a sentence to make it easier to understand. They also mimic the pauses introduced when speaking the sentence out loud.

So when you say:

A I thought could be eliminated because it doesn't enclose the non-essential information in a comma pair.

it suggests you've been taught a "rule" that simply isn't valid. It's a good practice to enclose nonessential information in a comma pair, because it helps the reader understand your logic, but it's not required.

For example, I could have written the previous sentence as:

It's a good practice to enclose nonessential information in a comma pair because it helps the reader understand your logic but it's not required.

Poor writing style, yes, but grammatically OK.

In your example sentence "living along the West Coast of North America" is an adjectival phrase modifying "otters". "In their habitat" is another such phrase modifying "kelp forests". You could exclude both without significantly affecting the main point of the sentence:

[Some] sea otters help keep [some] kelp forests healthy and vital.

It is grammatical to start the adjectival phrase with "that":

[The] sea otters that live along the West Coast of North America help keep ...

Answer choice C changes the entire sentence into a fragment because it combines everything after "otters" into a long adjectival phrase. We never find out what is known about the otters. A similar example:

The man living in that house and who helps his neighbor with her shopping.

The man ... does what? The sentence never resolves. Grammatical version:

The man living in that house helps his neighbor with her shopping.