How do you know when to use whom and when to use who?

In a sentence, WHO and WHOM have different syntactic functions.

Explanation:

WHO

WHO can be used as the subject of a sentence. For example, "who is your brother?" WHO can be replaced by he or she.

WHOM

WHOM is used as the object of the verb or object of a preposition as in "who is helping whom?" or "I know the person to whom the letter was addressed" respectively.WHOM can be replaced by him or her.

I would least likely to do : A

I would be most likely to do: C