Answer: The right answer is the D) Telling the truth.
Explanation: Just to elaborate a little on the answer, it can be added that the boy learned the importance of timing, since he learned "about not launching out too soon." In addition, he also learned about being careful, since he climbed "carefully, with the same pains you use to fill a cup up to the brim, and even above the brim," and about keeping his poise, that is, about remaining steady. There are no references about learning to tell the truth, so option D should be discarded.