Answer: A. Nominative.
Explanation: there are three pronoun cases in English: Nominative (or subjective), objective and possessive. The nominative case is when the pronoun acts as the subject of a sentence (this pronouns are I, you, he/she/it, we, they and who). The objective case is when the pronoun acts like the object of a sentence (they are me, you, him/her/it, us, them and whom). And the possessive case are pronouns that show that something belongs to someone (they are my, your, his, her, its, their, our, mine, yours, his, hers, ours, and theirs). In the given sentence the pronoun "I" is the subject of the phrase, so it is the nominative case.