Answer:
The correct answer is "motor unit - motor neuron".
Step-by-step explanation:
Motor unit and motor neuron are related terms, however they could not be used interchangeably because a motor unit is not comprised only of a motor neuron. A motor unit is made of a motor neuron, skeletal muscle fibers and neuron's axonal terminals. Therefore motor unit is a more broadly term, and is referred to the units that coordinate the contractions of single muscles.