Let's say that under normal conditions, your body produces 100 white blood cells per day (no idea if this is true , but bear with me). Then you get a fever, and now you make 200 WBC's per day. Then you take some asprin, your fever goes away, and you go back to producing 100 WBC's per day.

Big question: Is asprin inhibiting WBC production? No, it's just returning WBC production to normal. Rembember, "not stimulating" is NOT equivalent to "inhibiting" - there's something in between: neither stimulating nor inhibiting (i.e. normal conditions). Besides, it's the fever doing the stimulating, not the asprin itself. (C) doesn't work for this reason.

(D) is a problem because there's nothing in the premises to indicate scale. In other words, the problem tells us only that WBC's fight disease, but maybe there is a threshold number of WBC's before they can become effective, or after which they become ineffective. Furthermore, the severity of the disease depends on the type of disease (about which we have no info) as well as the WBC count.