Answered

If you write a sentence with an unclear antecedent, it means you used

Aan object pronoun instead of a possessive pronoun.
Ba plural pronoun when a singular pronoun was needed.
Ca possessive pronoun instead of a subject pronoun.
Da pronoun that could refer to more than one person, place, or thing.

Verified Answer :

If you write a sentence with an unclear antecedent, it means you used a pronoun that could refer to more than one person, place, or thing. 
For example: I never go to that place because they have stale bread.
Who does they refer to? We don't know, it is unclear. 

The anser is D. a pronoun could refer to more than one person, place, or thing.