Answer: B) Subject.
Explanation: A noun clause is a dependent clause that acts as a noun. Noun clauses begin with words such as how, that, what, whatever, when, where, whether, which, whichever, who, whoever, whom, whomever, and why. In the given sentence, the noun clause is "what books tell us" and it is acting as the subject of the phrase, because it is performing the action of the verb (should encourage).
