Which sentence contains a prepositional phrase?

a. with the proper , they'll complete the project early.
b. running all the way, he got there early.
c. they tried, although they didn't expect to win.
d. they sang it, then sang it again.

Aman lives on the twelfth floor of an apartment building. every morning he takes the elevator down to the lobby and leaves the building. in the evening, he gets into the elevator, and, if there is someone else in the elevator -- or if it was raining that day -- he goes back to his floor directly. otherwise, he goes to the tenth floor and walks up two flights of stairs to his apartment.

People really be on here lying to people about the right answer, dang

Which fits best in the empty box above?