Danielle wrote:B is not correct. This example question tests your knowledge of who and whom and when to use them. Here is the rule:

Who is always the subject of a sentence, i.e. Who let the dogs out?
Whom is always the object of the sentence, i.e. Mary did not know to whom to send the invitation

Here is another way to explain it that is cool and that I found on a
grammar website:

Rule Use the he/him method to decide which word is correct.
he = who
him = whom
Examples Who/Whom wrote the letter?
He wrote the letter. Therefore, who is correct.
For who/whom should I vote?
Should I vote for him? Therefore, whom is correct.
We all know who/whom pulled that prank.
This sentence contains two clauses: We all know and who/whom pulled that prank. We are interested in the second clause because it contains the who/whom. He pulled that prank. Therefore, who is correct.
(Are you starting to sound like a hooting owl yet?)
We want to know on who/whom the prank was pulled.
This sentence contains two clauses: We want to know and the prank was pulled on who/whom. Again, we are interested in the second clause because it contains the who/whom. The prank was pulled on him. Therefore, whom is correct.

Danielle wrote:Well, this is one is not so simple because it has the idiom 'one of'. In this case instead of he/him, you can use they/them. It's the same thing -- it will help you chose whether it is object of subject.

Presenters, one of _____ is blind. (they/them?)

Presenters, one of them is blind. Therefore use whom.