Answer: The correct answer for the fill in the blank is- C) Neurotransmitters.
Neurotransmitters are the chemical messengers in the body that help in the transmission of signals from one neuron to another neuron across a gap called synapse ( such as neuromuscular junction).
These chemicals are released from synaptic vesicles that are present in the nerve endings and also from some glands (like adrenal gland and pituitary).
Example of neurotransmitters- Dopamine, acetylcholine, serotonin.
