Since fireworks are danger many cities have laws preventing businesses from selling them.
Why is are danger incorrect? Could you please explain that, and how we can correct it?
Can we really count fireworks?
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Revised answer, in light of the comments by @Jonathan Garber:
Let's talk about two different senses of the word "fireworks":
If you're talking about a rocket or a firecracker, the singular of "fireworks" is still around, and it is indeed countable. At least some native speakers would say you can "buy a firework".
If you're talking about fireworks meaning "a pyrotechnic display", you always use the plural form "fireworks".
Let's talk about the second case. "Fireworks" used to be the plural of "firework", and at one time people used the phrase "a firework" in this sense.
These days, however, phrases like *"a firework" and ??"three fireworks" sound strange when referring to fireworks displays. The singular form is more-or-less lost in modern English, so we say could say the word is defective (lacks a form). That doesn't explain why ??"three fireworks" sounds odd; let's say that it's become a collective noun to explain that one.
So can you count "fireworks"? It depends on how you're using the word.
As for your sentence, it's fine, except for one thing: it should say "dangerous" rather than "danger". "Dangerous" is an adjective and can modify "fireworks" in this position; "danger" is a noun and cannot. (This has nothing to do with whether you can count fireworks.)
There are a couple of issues. For one, the phrase would normally have a comma after it.
Since fireworks are danger, many cities have laws preventing businesses from selling them.
Without the comma, the two parts of the sentence run together and are more difficult to parse.
As far as vocabulary is concerned, though, "fireworks are danger" equates "fireworks" and "danger" in a way that sounds far too strong to me. It basically means that fireworks and danger are one and the same, which (though not exactly incorrect) seems much too strong a relation to be appropriate here. (You might find it in anti-fireworks propaganda or something, but almost never in general usage.) Most people would say "Since fireworks are dangerous". (They might also say "Since fireworks are a danger", but that has its own problems. It partially implies that the danger is that fireworks might even be present.)
Actually, this has nothing to do with the countability of "fireworks". Yes, choice 'b' contains the error in the sentence, but the problem isn't the word "are", but the word "danger".
In a construction like "[X] is/are [Y]", Y needs to be either an adjective/adjectival phrase:
Lions are dangerous.
My kitten is dangerously cute.
or a noun/noun phrase preceded by an article:
The school mascot is a lion.
Cute kittens are a menace to society.
Thus, your example sentence needs to become either
Since fireworks are dangerous ...
(using an adjective); or
Since fireworks are a danger ...
(using an indefinite article plus a noun).
Just for completeness, I'll mention that although we use fireworks both to mean the concept in general (uncountable) and the pyrotechnic devices (countable), it is not actually incorrect to use firework singular, if the context demands it.
After successfully setting off a sequence of firecrackers, rockets, and fountains, he was left with a single firework that simply refused to light.