Answer:
B) Neither the puppies nor our full-grown dog likes their new toys.
Step-by-step explanation:
In this sentence, the pronoun "Neither" refers to a selection between two subjects: The puppies (plural subject) and our full-grown dog (singular subject). Since this pronoun can only refer to only one of the subjects and the one that comes second is of singular number, the pronoun's number is singular as well.
The antecedent "their" must then be replaced by one with singular number. In this case, it must be replaced with "its" as it refers to an animal.