Answer:
B. above the desk
Step-by-step explanation:
A prepositional phrase consists of the preposition, its object which could be a noun or pronoun, and any other modifiers such as an article. This kind of phrase is used in modifying a verb, and a noun. A preposition is used to indicate time, place, direction, and location of an object. The preposition in the phrase is "above" because it indicates location while the object is "desk". The modifier and article in the sentence is "the".
So the combination of these three parts of speech make up the prepositional phrase.