Answer
b. the textual clue that refers to the pronoun
A pronoun antecedent refers to the word that comes before the pronoun. The pronoun has to agree with the noun especially in number since it is the part of speech that replaces the noun. In other words, when the pronoun is singular then that means that the antecedent must also be singular however if the pronoun is plural so do with the antecedent. In determining whether the antecedent is singular or not, it is very important to know what words between the antecedent and the pronoun to ignore.