The correct answer is d. a pronoun that could refer to more than one person, place, or thing.
An example of an unclear antecedent is:
Jim and Billy were going to the park until he tripped on the curb and sprained his angle.
The unclear antecedent is he. Who does he refer to? Does it refer to Jim or to Billy? To make it a proper antecedent, you would need to replace he with Jim or with Billy.
