The correct answer is number of the antecedent.
Number of a noun/pronoun can be either singular or plural. Antecedent is the word in front of a pronoun/phrase that the pronoun/phrase refers to. So, if you insert a phrase between the subject and the verb, that phrase won't affect the number of the antecedent. For example:
The bottle of Coke is on the table.
The phrase of Coke didn't affect the antecedent's (the bottle) number.
