Hi @Zahra, thanks for your post! Happy to explain.
This is an Argument Exchange / Point at Issue question. Let's first be sure we are clear on the stimulus.
Oscar's conclusion is that "student evaluations provide the best assessment of teacher performance."
Bettina is not so sure, because "students generally fail to appreciate the impact of a teacher until many years later." So, she argues that "peer evaluations of teachers might be a good supplement or a better alternative." This means that Bettina does not believe that student evaluations are the "best assessment."
Answer choice (C) articulates this point of disagreement.
Answer choice (A) is not textually supported. Even Bettina thinks that students can sometimes adequately judge a teacher's performance - she just thinks this may not happen "until many years later." That's not "never."
Hope this helps! Please let us know if you have any additional questions.