What structure most directly stimulates a skeletal muscle fiber to contract?

The right answer is motor neuron.

A motor neuron is a nerve cell that is directly connected to a muscle and controls its contraction. Motor neurons are the exit route from the central nervous system or the final route of any motor act. The cellular bodies of the motor neurons are located either in the brainstem or in the ventral horn of the gray matter of the spinal cord.

the electromagnetic spectrum includes, from longest wavelength to shortest: radio waves, microwaves, infrared, optical, ultraviolet, x-rays, and gamma-rays.