D is obviously the answer that the authors intend to be the correct one. But it's a really poorly written question. As you point out, it should actually be two sentences. Additionally, since "computers" is a plural subject, the predicate nominative should also be plural. It should say "Computers are (useful) tools." And the phrase "in serving the people" is extremely awkward and doesn't add any information. Finally, the second sentence has a singular pronoun that's replacing the plural subject of the first sentence. It should say "(a command of) them."

D

D