When to use "one of"?

Question

Although the question is about possessives, I got it wrong because I am confused whether to use one of or not.

The correct answer is D. I marked C because I thought that D is awkward and wordy.

Please help me with this concept.

Answer 1

(C) is wrong because you cannot say:

• *Mike Tyson (singular) was boxing's most fearsome competitors (plural).

You need the partitive "one of" to make the subject complement singular.

If Tyson was considered the most fearsome boxer of all, then you could say:

• Mike Tyson was boxing's most fearsome competitor.