Welcome to the forums! Good question.
This question uses conditional reasoning. It's somewhat difficult to follow, but if you isolate the conditional statement, the additional premise, and the conclusion, it might be easier to grasp:
- P1: Some intro science courses are designed so that only committed students will pass.
We could symbolize this statement like this:
- P1: Successful Intro Course Only Committed Students Pass
Now what else do we know?
- P2: Unenthusiastic students sometimes pass.
Conclusion: These classes are not successful.
From the contrapositive of P1, we know:
- P1: Not Only Committed Students Pass Not Successful Intro Course
Where is the missing link here? What is the author assuming? The author assumes that unenthusiastic students are not committed students.
- Assumption: Not Enthusiastic Students Not Committed Student
This is the missing link. For this author's argument to make sense, we must assume that the unenthusiastic students are not committed to science majors. This corresponds to Answer Choice (D).
Let's take a look at Answer Choice (C).
Remember that in an Assumption question, we're looking for a statement that is a necessary but unstated belief of the author's, something the author must believe in order for the argument to make sense, so for Answer Choice (C), we should ask ourselves:
- Must the author believe some of the enthusiastic students don't pass?
This statement seems to be in sync with the author's implicit assumption that enthusiasm and commitment are related to each other; however, the author's argument doesn't concern what happens with the enthusiastic students but instead with the unenthusiastic students. The author argues that the classes are unsuccessful because some unenthusiastic students pass. We have no argument here about what happens to the enthusiastic students. It stands to reason that if the author equates enthusiasm to commitment, then he or she would expect some of these enthusiastic/committed students to pass. However, again there is no necessary expectation that every enthusiastic/committed student must pass.
Following the author's logic, the classes could still be successful even if some committed students fail. Therefore, Answer Choice (C) is not an assumption.
Does this make sense?