This question is testing PARALLELISM and MEANING.

In laboratory rats, a low dose of aspirin usually suffices to block production of thromboxane, which is a substance that promotes blood clotting, but not seriously interfering with the production of prostacyclin, which prevents clotting.

(A) which is a substance that promotes blood clotting, but not seriously interfering

The word BUT must link two parallel ideas, showing a contrast in meaning between them. The contrast needed is "it suffices to block X but does not interfere with Y." "But not interfering" is not parallel to "suffices." In fact, it seems to be parallel to "clotting," which is illogical. Furthermore, there is no need to use a dependent clause "which is a substance" as a modifier when we could simply use the appositive "a substance."


(B) a substance that promotes blood clotting, but not seriously interfering

It's more concise than A, but it still doesn't fix the parallelism issue. Incorrect.


(C) a substance that promotes blood clotting, but does not seriously interfere

Concise modifier and logical parallelism. Correct.


(D) which is a substance to promote blood clotting, but does not seriously interfere

The verb "interfere" is parallel, but the concision issue remains. Furthermore, "a substance to promote" implies intention - that would add meaning not present in the original. Incorrect.


(E) which is a substance that promotes blood clotting, but not a serious interference

"Not a serious interference" is not parallel to any preceding idea. Incorrect.


The answer is C.