The correct answer is C) Without proof of challenging courses, colleges might disregard our transcripts.
Step-by-step explanation:
In an argument, the last sentence needs to support the main position of the author by restating the main point. In the case of the sentence "I mean, if we can’t show that we took tough courses, colleges will turn us down" this supports the main position of the author about the need for more advanced placement classes. However, the use of "I mean" "though" or "can't" is too informal and not appropriate.
Due to this, the same idea should be restated using formal language, in this way, "Without proof of challenging courses, colleges might disregard our transcripts" is the best option, because this preserves the essence of the idea but uses formal language.