Answer: The correct answer for the blank is- d. inhibitory neurotransmitter.
Neurotransmitters are the chemical messengers in the body, which are released by the axon terminal of neuron ( also called nerve cell) and transmit nerve impulse to the neighboring cell ( which could be a muscle cell or a nerve cell) .
There are primarily two types of neurotransmitters that are-
1) Excitatory neurotransmitter and 2) Inhibitory neurotransmitter.
Inhibitory neurotransmitter have inhibitory effects on the neuron that is they reduce the chances that a neuron will fire an action potential. In other words, they are responsible for slowing signals between neurons.
Example- The most common inhibitory neurotransmitter is GABA ( gamma amino butyric acid)