As always we should start by reading the question first. Once we see that it is a weaken question, we can read the argument with a suspicious mindset, which will help us to find the best answer.

What's the conclusion? It is this: Marc must have learned less about algebra last year than most other students in our class. Oftentimes conclusions contain a modal verb (such as should, must, or will) and this feature can help us find the conclusion more quickly.

Why? Why do we think that Marc must have learned less about algebra than did the other students? Because he answered only 75% of the questions correctly on last year's Algebra I final exam.

Does that really mean that he learned less? At the time of the exam perhaps he knew less, but unless we assume that he started at the exact same level as the other students, we cannot assume that his end point indicates all by itself the amount of progress he has made over the last year. The argument assumes that he did start at the same level as the others.

Once we know the assumption, we can predict the right answer. Somehow the best answer will say that he did not have the same initial starting knowledge as did the other students. Which of the answers gives us that information? Answer choice (B) by suggesting that most other students took an introductory level algebra course, strongly suggests that he started at a lower level of algebra knowledge than did the other students. That's why (B) is the best answer.