A prepositional phrase has to start with a preposition, and the only option here which starts with one is B, 'during the darkest night', where 'during' is a preposition.
During is a preposition. It gives the relation between night and whatever is happening. A prepositional phrase begins with a preposition and ends with a noun. All of the other options begin with action verbs, things you can do. These are not prepositional phrases.
