Harvey takes the drug viagra. how does the drug produce erections for harvey?

The answer to this question would be: B. It relaxes blood vessels in Harvey’s penis, allowing them to expand. 

Viagra active component is sildenafil citrate which has the ability to relax the smooth muscle in the artery and dilate the arteries of the penis. As the arteries in penis dilate, the blood flow into penis will be increased. This will make penis filled with blood and give pressure to nearby veins causing them to close. All this event cause increased pressure in penis that was making the erection