The nominative case is used for noun & pronoun. It is used when a noun or a pronoun is used as the subject of a verb.
This case is also used to express subjects completing an action.
Here A) is the appropriate answer.
Who is the subject completing the action of help.
B) Whom - is used when referring to the object of the sentence
C) Whose - is used for lifeless things and cannot be used here.
D) Which - is used for animals and lifeless things