What is the term for something that has a subject and a verb but does not have to be a complete sentence?

Which line from “a simple way to create suspense” best supports the central idea that writers should not give their readers answers too quickly? a. the reader learns to chase, and the momentum becomes unstoppable. b. so writers are taught to focus on ingredients and their combination. c. we had to invent a solution to the serious problem it posed. d. thus the principle works in a micro sense, as well as in a macro one.