Question

What ion is entering the axon terminal at A, and what effect does it have?

Answer 1

The ion which is entering axon terminal A is called Ca²⁺.
The effect which it has is, it causes the release of neurotransmitter from the axon terminal.

Axon terminal is termed as distal termination of telodendria of an axon. Another name for the axon terminal is synaptic boutons. Axon is a long, slender projection which is of a nerve cell. 
Their work is to conduct electrical impulses which are referred to as action potential which is a way of neurons of the cell body. Neurons use electrochemical signals and also a neurotransmitter chemical which transmits impulses from one neuron to another.
Axon terminals are being separated from neighboring neurons by a gap known as a synapse. Both the axon terminal and neuron they come from a point which is called the presynaptic neuron.

Answer 2

Let me try to explain this. The ion entering this axon terminal at A is the sodium ion better known as Na+. This ion causes an effect called repolarization of the axon terminals which is an electrical activity which cause muscular activity