Total internal reflection is the phenomenon employed in waveguide operation.

Explanation:
Waveguides are structures that guide electromagnetic waves, such as microwaves or light waves, along their length. They are used in various applications, including communication systems, radar systems, and optical devices.

When an electromagnetic wave travels through a waveguide, it encounters the boundary between the waveguide and the surrounding medium. At this boundary, the wave can undergo different phenomena, such as reflection, refraction, and total internal reflection.

Reflection:
Reflection occurs when an electromagnetic wave encounters a boundary and bounces back into the original medium. It can happen at the waveguide interface, but it is not the main phenomenon employed in waveguide operation.

Refraction:
Refraction is the bending of an electromagnetic wave as it passes from one medium to another with different refractive indices. This bending occurs due to the change in the wave's velocity. While refraction can occur at the waveguide interface, it is not the primary phenomenon employed in waveguide operation.

Total Internal Reflection:
Total internal reflection occurs when an electromagnetic wave encounters a boundary and is completely reflected back into the original medium. This phenomenon happens when the wave is traveling from a medium with a higher refractive index to a medium with a lower refractive index and the angle of incidence exceeds the critical angle.

In the context of waveguides, the waveguide structure is designed to have a higher refractive index than the surrounding medium. This ensures that when the wave reaches the boundary between the waveguide and the surrounding medium, it undergoes total internal reflection. This reflection allows the wave to propagate along the length of the waveguide without significant loss of energy.

By employing total internal reflection, waveguides can confine and guide the electromagnetic waves, preventing them from spreading out and losing their energy. This makes waveguides efficient in transmitting and manipulating electromagnetic waves.

So, the correct answer is option 'C - Total internal reflection'.

Total internal reflection