Answer:
I assume that you were given several options. You haven't provided them and they are unavailable online, but I will try to help you anyway.
The given sentence tells us about how important gravity is. The tool used to emphasize its importance is parallelism.
Parallelism (also referred to as parallel structure or parallel construction) refers to the repetition of the same or similar grammatical structure in clauses and sentences. Its purpose is to create a rhythm, make a comparison, emphasize, or elaborate on an idea.
The given sentence consists of four clauses that begin the same way - with the phrase it keeps. This phrase actually represents the subject and the predicate. Every time, it's followed by the object (in this case, the recipient of the action expressed by the verb keeps). The purpose of parallelism here is to emphasize the importance of gravity.