it is very confusing to me, but I'd pick D.
A Even after long periods of sleep when adenosine is at its lowest concentration in the brain, the number of brain cells bound with adenosine remains very large. - weakens the argument. WRONG
B Caffeine, which has the effect of making people remain wakeful, is known to interfere with the binding of adenosine to sites on brain cells. - YES. CORRECT
C Besides binding to sites in the brain, adenosine is known to be involved in biochemical reactions throughout the body. - Irrelevant. WRONG
D Some areas of the brain that are relatively inactive nonetheless release
some adenosine. - Irrelevant (relatively inactive?? what is relatively?). WRONG
E Stress resulting from a dangerous situation can preserve wakefulness even when brain levels of bound adenosine are high.-Out ot scope. WRONG.
Has to be B.