The passage starts by telling us that maybe it's a good idea to teach high school kids calculus. Okay, let's explore. Is it a good idea?
Well, it might "benefit them" but it didn't specify in what way. So some unspecified benefit on the one hand.
Then, the passage turns around and tells us that there's some "level of abstraction" involved in calculus. Okay, like a high level or a low level? Don't know. But, if these high school kids aren't ready for whatever that "level of abstraction" is, then they may "abandon the study of mathematics".
So, if we're going to teach them calculus, we better make sure they're ready to handle that "level of abstraction".
Why? Because if they aren't ready, they might abandon the study of math. I mean, god forbid they decide to take up acting or some such non-sense.
Okay, I'm kidding, but you see the assumption right?
The assumption is that we don't want them to abandon the study of math. In other words, teach math to students only if it won't lead the students to abandon it. In other words, if you introduce calculus to students, then make sure that they can handle the "cognitive challenges" (or "level of abstraction") "without losing motivation" (or "without abandoning it"). That's (A). (A) tightens up the space between the premises and conclusion.
(C) is problematic for two reasons. First, is calculus a "cognitive task that requires exceptional effort"? We don't know. So we have to presume that it is. Okay, that's bad enough.
But, even if we presume that it is. Then all (C) tells us is that it undermines the motivation of those who attempt them. In other words, calculus just straight up hurts your self esteem and motivation. Never mind be ready to handle the "level of abstraction". It just hurts you. So... how does this help our argument?