Increasing the amount of action potentials that arrive at the presynaptic terminal will:

a) increase the amount of depolarization in the terminal
b) increase the amount of ca2+ that enters the presynaptic terminal.
c) increase the number of vesicles that fuse with the presynaptic membrane
d) increase the amount of neurotransmitter released.

Increasing amount of action potentials that arrive at pre synaptic terminal will increase the amount of Ca2+ that enters the presynaptic terminal.

Explanation:

Neuromuscular junction is called as the site of signal exchange which occur in step wise manner.As action potential reaches the axon terminal, it opens axon terminal through which calcium enters.Then fusion takes place between presynaptic membrane that causes release of ACh in synaptic cleft through exocytosis.Binding of ACh with postsynaptic receptor cause opening of ion channels which allow the entry of sodium and potassium.This flow generates action potential that travels to the myofibril and causes muscle contraction.