Monique cannot seem to sit still, talks rapidly, and never seems to get tired at the end of the day. Her friends suspect that she is taking:

A. cocaine.
B. Rohypnol.
C. LSD.
D. speed.

cocaine

To determine the answer to this question, we need to understand the symptoms and effects of different substances regarding Monique's behavior.

The symptoms mentioned - inability to sit still, rapid talking, and lack of tiredness at the end of the day - can be associated with the use of stimulant drugs. Stimulant drugs are substances that increase activity in the central nervous system, leading to increased alertness, energy, and focus. Some examples of stimulant drugs include amphetamines, methamphetamines, and cocaine.

Out of the given options, cocaine is a stimulant drug that can produce the mentioned effects. Cocaine is a powerful stimulant that increases levels of dopamine, a neurotransmitter associated with pleasure and reward, in the brain. This leads to a surge of energy, increased heart rate, talkativeness, and excessive movement.

Therefore, the correct answer is A. cocaine.