Answer:
b. Motor Unit
Step-by-step explanation:
A motor unit is consist of motor neuron and muscles fibers connected through the axon terminal of neuron. Motor unit function for the movement (contraction and relaxation) of muscles.
A neuron is connected with muscles fiber through the axon terminal of neuron and is called neuromuscular junction. Skeletal muscle fiber is consist of a cylindrical muscle cell that contain microfibrils which participate in contraction and relaxation of muscles.
Neuron and muscle fiber coordinate with other other for the proper functioning of motor unit.
Hence, I selected diagram "b" that represents motor unit.