(A) can help us reach a conclusion that parents should encourage children to be happy about something. But we’re trying to prove that children should NOT encourage children in a certain way. Encouraging children to be happy about something doesn’t help us prove that children should NOT be encouraged in a certain way.

(B) can help us reach a conclusion that parents should try to make sure their children have at least some desires that are easy to satisfy. But it doesn’t help us prove that children should NOT be encouraged in a certain way. (B) allows parents to encourage children to value outdoing others, because even though that makes happiness difficult to achieve, (B) just commands parents to make sure there’s some other desire that children have that’s easy to achieve.

Builds a bridge from the premises to the conclusion. We know from a premise that having the trait of valuing outdoing others makes a person less happy. (C), then, would allow us to conclude that one should never encourage another person to have the trait of valuing outdoing others. So parents shouldn’t encourage children to value outdoing others.

Does putting value on outdoing others help one have significant achievements? Maybe. And if so, (D) would actually support the opposite of the conclusion.

This tells us about the comparative importance of one’s achievements relative to different metrics. So how much a child achieves relative to their own potential is just as important as how much they achieve relative to others. But even if that’s true, we still don’t know why parents shouldn’t encourage children to value outdoing others. Sure, the child’s achievements relative to their own potential is also important...but why isn’t beating other people something we should value? A child can value both beating other people and maximizing one’s achievements relative to potential.