Jeff's drug addiction to cocaine occurred because cocaine overstimulated the reward system in his brain and disturbed the function of which neurotransmitter?

Dopamine.

Explanation:

Cocaine is used as a recreational drug and addictive in nature. It can be smelled or directly inhaled in the veins. This drug causes the feeling of restless and happiness.

The dopamine is the important neurotransmitters that regulates the mood and other biological activity of the body. The cocaine inhibit the reuptake of dopamine and leads to its accumulation in the brain. This might also break the blood brain barrier.

Thus, the answer is dopamine.