In the sentence "What is your favorite subject?" the word subject is a A) noun. A noun is a person, place or thing and in this sentences the word subject would be a thing, thus a noun. In fact the word subject is never a verb.
The word subject should have its accent on C) sub, the first part of the word and more specifically on the letter U. As a general rule the accent mark on a two syllable word usually goes on the first syllable and thus should be sub.