Answer: C) It is a dependent clause.
Explanation: an independent clause is a sentence that has complete meaning on its own (it has at least a subject, a verb and all the necessary complements), a dependent clause is the contrary, is a phrase that, in order to make sense, it needs to be with an independent clause. The given excerpt is a clear example of a dependent clause, because as we can see, it doesn't have complete sense, we can tell that it is missing something.
A. They can undo complicated bolts on gates.