Which sentence contains a prepositional phrase?
a. they sang it, and sang it again.
b. they tried, although they didn't expect to win.
c. with the proper , they'll complete the project early.
d. running all the way, he got there early.

The answer is C: "With the proper help, they'll complete the project early."

The word "with" is a preposition because it expresses a relation to the completion of the project. The rest of the phrase answers the question "With what?" Thus, "With the proper help" describes what conditions must occur in order for the project to be completed early.