If the axon terminal synapses with a skeletal muscle cell, we know the neurotransmitter is what? Is it acetylcholine, norepinephrine, epinephrine, serotonin, or can it be any of these?

it is acetylecholine

when action potential reach axon terminal , activation of Ca channel occur leading to Ca influx with rupture of vesicles and release of acetyle choline at the motor end plate to stimulate nicotinic sodium channel on muscle membrane leading to influx of Ca and contraction of muscle