Final answer:
Dopamine is the neurotransmitter associated with both addiction and psychosis, as it plays a significant role in the brain's reward system and its imbalance can lead to such psychiatric conditions.
Step-by-step explanation:
The neurotransmitter associated with psychosis or addiction (Cluster A) is dopamine (b). Dopamine is key in the brain's reward system and is related to pleasure, addiction, and psychosis. Excessive levels can be linked to these conditions. Drugs like cocaine and amphetamines increase dopamine levels, leading to addiction due to their effects on reward pathways. Additionally, an imbalance in dopamine is implicated in schizophrenia, characterized by psychosis.