Answer:
Option D - Once initiated, an action potential is propagated the entire distance from the upper motor neuron to the muscle cell.
Explanation :
What happens when action potential is initiated is that it travels to the nerve ending where it propels the release of neurotransmitter.
Only action potential travels down to the next neuron, in a case where the released neurotransmitter is excitatory, otherwise, the action potential won't travel beyond if the neurotransmitter is inhibitory .