Answer: a. To the cabinet.
Step-by-step explanation:
The prepositional phrase refers to the group of words which is made up of the preposition (such as to, into, before, in, at, of etc), its object, and the words which also qualifies the object. A prepositional phrase typically modifies either the noun or the verb.
The prepositional phrase in the sentence "Please return the book to the cabinet" will be "to the cabinet". Here, the preposition is "to" and it modifies the noun " cabinet".
Therefore, the correct option is A.