Answer: b.

Explanation:
Whom is the correct choice for the true grammar stickler. As the object of the sentence, whom is the correct form. As the subject, it’s who.

However, I’ll go out on a limb here and suggest that it’s time to let this one go in most situations. When’s the last time you heard someone use whom in casual conversation? Formal writing is another story — there’s still a use for whom there. But for everyday use, I think it’s anachronistic. Nevertheless, if you’re going to use whom, you want to do it the right way. There’s nothing much that makes you look sillier than using a fussy, fusty word incorrectly.

@GrammarGirl Mignon Fogarty explains who versus whom in much greater detail. She also offers this example of bad grammar:

I know, it’s shocking, but the Rolling Stones were being grammatically incorrect when they belted out the song “Who Do You Love?” which I think was originally written by Bo Diddley.

Ha. One suspects that “Whom do you love?” couldn’t pack the same rock ‘n’ roll oomph as “Who do you love?”

There’s a place for whom, but that place is getting smaller all the time. IMO, it belongs in formal writing and, possibly, some rarified circles of society — but I wouldn’t know about that. Who uses whom in writing anymore, let alone in everyday speech? If you say, for example, “Whom do you wish to speak to?” (but let’s face it — someone who uses whom is more likely to say “To whom do you wish to speak?”), odds are, whoever is calling will think you are a prissy stick.

The situation is much worse if you use whom incorrectly: People who don’t recognize the error will think you’re a prissy stick and people who do recognize it will think you’re a dimwit AND a prissy stick.

My advice? Use whom correctly when you can’t avoid it — but avoid it when you can.

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