Correct option is C
The Transverse Electromagnetic (TEM) mode is supported in coaxial cables but not in standard hollow rectangular waveguides.
Coaxial cables have two conductors (an inner and an outer shield), which allows for the presence of both radial electric fields and encircling magnetic fields that are both entirely transverse to the direction of propagation. The TEM mode has no cut-off frequency and can propagate from DC up to a certain high frequency limit.
Rectangular waveguides are hollow structures with only one conductor. The boundary conditions in such a single-conductor hollow guide make the existence of a pure TEM mode impossible. They can only support Transverse Electric (TE) and Transverse Magnetic (TM) modes, which have a non-zero component of either the magnetic or electric field, respectively, in the direction of propagation.
