An ambiguous reference occurs when a pronoun

a. may refer to two or more antecedents

b. refers to an unknown antecedent

c. modifies an adjective, adverb, or verb

d. functions as the object of a preposition

A pronoun is used to refer to a particular antecedent only in the sentence and this antecedent should be a single noun. But when the pronoun refers to multiple antecedents and the sentence and it becomes unclear to which antecedent, this pronoun is referring to, this is called an ambiguous pronoun reference. Therefore, the correct answer is option A. May refer to two or more antecedents.