Answer choice A

Additionally, if someone could explain why answer choice B is incorrect that would be very helpful. Thanks in advance.

@shafieiava, Let's look at (A) and (B). (B) says, "Only those parts of university-level mathematics that are the most concrete should be taught to preuniversity students." The problem with (B) is that it does not apply to the argument because the author qualifies her conclusion by saying, "if we're going to teach pre-university students calculus." It's possible that the author could agree with (B) but still offer advice about what to do if you ignore this recommendation and teach something abstract regardless. Thus, we can get rid of (B). (A) says, "Only those who, without losing motivation, can meet the cognitive challenges that new intellectual work involves should be introduced to it" (A) basically means that being able to meet the mental challenges of new work is essential for an introduction to that type of intellectual work. Although it has very general language, (A) does make the mathematics teacher's argument stronger. Abstract thinking is a certain type of mental challenge, and calculus qualifies as intellectual work, so (A) strengthens the author's argument by providing us with a strong premise that helps justify what the teacher is saying. Thus, (A) is the correct answer choice. Hope this helps. Let us know if you have any other questions!