At first, I could not find the option with an unclear antecedent at all. The way you posted them, all the options are correct. Then I looked this question up online and realized the reason why all your options are correct is because in option C, where the pronoun should be "it", it was changed for "them". This is the question I found:
Which sentence contains a pronoun with an unclear antecedent?
A. Several beach-goers got caught in the riptide when they went swimming.
B. The puppy and kitten played together while they ate.
C. Perry dropped his cheese and apple on the floor but then ate it.
D. The pilot said he would alert the crew when it was time to land.
Answer:
According to the question above, the option that contains a pronoun with an unclear antecedent is:
C. Perry dropped his cheese and apple on the floor but then ate it.
Explanation:
Perry dropped two things - "cheese" and "apple". He then ate "it", but which one? Did he eat the cheese, or the apple? It's impossible to know. The pronoun "it" does not have a clear antecedent. The sentence does not contain enough information for us to infer that he ate the cheese, for instance, but not the apple. For this reason, the best choice is:
C. Perry dropped his cheese and apple on the floor but then ate it.
