The grammatical case used for the subject of a verb, as “he” in “he runs,” is the nominative case.
The nominative typically marks the person or thing performing the action of a verb, or the subject being described. In “he runs,” “he” is nominative because it is the subject of “runs.” In English, the distinction is easiest to see in pronouns: “I,” “he,” “she,” “we,” and “they” are subject forms, while “me,” “him,” “her,” “us,” and “them” are object forms.
English has lost much of the noun-case system found in languages such as Latin, German, Russian, and Ancient Greek. Word order now does much of the grammatical work: “The dog chased the cat” differs from “The cat chased the dog.” English still preserves nominative and objective contrasts in pronouns, however. The verb “to be” also creates a traditional complication, as in “It is I” versus the widely used “It is me.”
The other options name different cases or rhetorical terms. Accusative generally marks a direct object, dative an indirect object or recipient, and genitive possession or close association. The nominative is therefore the standard canonical answer for the subject case.