Serotonin is produced by the _____.

Correct option is B

Serotonin is a neurotransmitter synthesized by neurons in the raphe nuclei of the brainstem. These nuclei send serotonergic projections throughout the central nervous system and regulate mood, sleep, and appetite. Deficiency of serotonin is linked with depression and anxiety disorders. The other listed structures are associated with different hormones or neurotransmitters and not serotonin.
Explanation of options: (a) ✘ Beta cells – Located in the pancreas, beta cells are responsible for insulin secretion, not serotonin. (b) ✔ Raphe nuclei – Found in the brainstem, these nuclei are the main source of serotonin in the brain. (c) ✘ Alpha cells – Also located in the pancreas, they produce glucagon, a hormone that increases blood glucose. (d) ✘ Substantia nigra – This brain structure is known for dopamine production, essential for movement control.