Which of these is a neurotransmitter?

A) Monoamine oxidase: Monoamine oxidase (MAO) is an enzyme, not a neurotransmitter. It is responsible for breaking down certain neurotransmitters, such as dopamine, serotonin, and norepinephrine, in the brain and other parts of the body. While it plays a crucial role in regulating neurotransmitter levels, it is not itself a neurotransmitter. B) Cholinesterase: Cholinesterase is also an enzyme, not a neurotransmitter. It breaks down acetylcholine (ACh) at synaptic junctions to terminate its action after it has transmitted a nerve impulse. This enzyme is important for the proper functioning of cholinergic synapses but does not function as a neurotransmitter. C) Acetylcholine (ACh): Acetylcholine (ACh) is a neurotransmitter. It is released by nerve cells at cholinergic synapses and plays a key role in both the peripheral and central nervous systems. ACh is involved in transmitting nerve impulses to muscles (muscle contraction) and is also important in cognitive functions like memory and learning in the brain. D) Calcium: Calcium is a vital ion involved in many cellular processes, including muscle contraction and neurotransmitter release. However, it is not a neurotransmitter. It plays a role in the function of neurotransmitters but does not act as one itself.