Hey adrz24,

Right off the bat, we can rule out D because it uses past tense verb "was" - "students will take ethics seriously if it was taught differently" doesn't make any sense.

However, this still leaves us with C. Looking at our original, we see that this sentence says:

"students will take ethics seriously only if it would be taught as a separately required course"

Ignoring the grammatical issue, we see that the intended meaning is "students will only take ethics seriously if it is is separately required".

On the other hand, C says:

"students will take ethics seriously if it is taught only as a course required separately"

Moving the word "only" a few words changes the meaning here pretty dramatically - now the sentence tells us "students will take ethics seriously if it is only separately required ... but there may be other ways to get students to take ethics seriously".

On the GMAT, we always want to match the intended meaning of the original sentence (as long as the sentence itself is logical). So the change in meaning in C allows us to eliminate. Similarly, if we weren't able to eliminate D based on the tense issues, we would find that D similarly changes the meaning of the sentence and can be eliminated.