Why? Because if students are taught calc before they're geeky enough, they may abandon studying math altogether.
(Sound familiar? It's probably what happened to most of US!

The gap here is the assumption that if something leads to some folks abandoning it, we shouldn't do it. Or, as (A) words it, we should introduce math only to those who can be smart enough and keep up their motivation. Why not expose the less geeky kids to calc and let them drop out?
(B) is out of scope - we're not comparing various courses, and who says calc is university level?
(C) is tempting, but we already know that calc. can undermine the motivation of some people - it doesn't help the argument to know that this is a general issue with cognitive tasks. This is a premise-booster. It doesn't help us conclude what we should do know that we know some kids might be adversely affected. Plus, we don't care if its because of the effort or because of some other reason, such as a chemically-derived neural meltdown from exposure to an idea that is too complex.
(D) is out of scope - application? And, again, is calc university level?
(E) weakens the argument - we do want to consider the level of abstraction and its effect on students!
As to the question above about whether an answer is wrong if it introduces new terms: no. Particularly strengthen, weaken and necessary assumption questions may do this. But, it's good to be on the lookout for this sort of thing!
#officialexplanation