Final answer:
Cocaine is an agonist for the dopamine neurotransmitter because it increases dopamine levels in the brain by preventing reuptake and stimulating more release.
Step-by-step explanation:
The recreational drug cocaine is an agonist for dopamine neurotransmitter. Cocaine increases the levels of dopamine in the synaptic cleft by impeding the reuptake of dopamine by the nerve cells and causing more dopamine to be released. This action results in heightened stimulation of the pleasure centers of the brain. Due to its mechanism of amplifying dopamine activity, cocaine would be classified as an agonist, not an antagonist, because it enhances the actions of a neurotransmitter rather than blocking it. Therefore, the correct answer is A. dopamine.