In the passage from "We Are Not Yet Equal," the writer varies syntax primarily by including a mix of sentence structures rather than relying on one specific type. However, to answer your question more specifically regarding the provided responses, the most accurate choice based on the passage is that the writer does not predominantly use imperative sentences, complex sentences, or interrogative sentences. Instead, the passage contains mostly declarative sentences that are straightforward in nature.
Given the options available, the correct answer would be:
by using mostly simple sentences.
This choice reflects the factual and direct nature of the statements made in the passage, which focus on the historical context without complex sentence structures.