Communication across a synapse is initiated by the release of a neurotransmitter from the region of the neuron.

Axon terminal.

Explanation:

Neurons are also known as nerve cells. They are the fundamental unit of nervous system, and brain. They communicate with each other with the help of electrical events know as neurotransmitters, and action potentials.

Axon terminal is the end of the branch of nerve axon, it is a long slender nerve fiber which helps to conduct electrical signal to the gap between nerve cells known as nerve synapse. With the help of neurotransmitter the signal then move to the synapse to another axon.

option a is correct i.e.

explanation:

in context to the membrane of nerve impulse, the outer membrane of neuron is semi permeable that is only sodium ions can pass through the membrane i.e. smaller ions can pass through the membrane while larger ions cannot pass through it.