Drug cravings become heightened
a. as soon as the user wakes every morning.
b. in the evenings.
c. whenever the user is exposed to any situation that triggers a memory of the drug use.

The answer is letter C.

Drug craving become heightened whenever the user is exposed to any situation that triggers a memory of the drug use. Often times, people who undergo drug rehabilitation are taken away from their most common environment where drug abuse is prominent. This is the reason why many drug users who leave rehabilitation centers have the tendency to return to their old ways. Their exposure to the outside world triggers drug cravings and eventually the cycle keeps repeating itself.