A motor unit is

a. one motor neuron

b. one motor neuron and the group of muscle fibers it innervates

c. several motor neurons and the group of muscle fibers it innervates

d. one motor neuron and only one muscle fiber

one motor neuron and the group of muscle fibers it innervates.

Explanation:

The concept of motor unit was given by Charles Scoot. A motor unit work together in a group and result in the coordination of a single muscle.

A motor unit is made upof a motor neuron and the skeletal muscle fibers that innervates the motor neuron. The motor unit consists of same muscle fibers.

Hence, the correct answer is option (b).