The correct answer is option d, that is, neurotransmitters.
A neurotransmitter refers to a chemical component, which is discharged at the terminal of a nerve fiber by the introduction of a nerve impulse, and by diffusing across the junction or synapse. It helps in the transfer of the impulse to another muscle fiber, nerve fiber, or some other composition. Acetylcholine is a kind of neurotransmitter, that is, used by neurons in the CNS and the PNS in the control of activities ranging from heart rate and muscle contraction to digestion and memory.