ANS:C - TE and TM waves

Answer : Option D.

A waveguide is a single conductor with a dielectric inside it (Air is also a dielectric).

TEM mode is characterized by electric fields and magnetic fields perpendicular to one another and perpendicular to the direction of propagation.

In order to have such a configuration, there has to be a source of electric field at the centre from where E fields originate and terminate on the outer conductor and there has to be a current source which can generate magnetic fields.

For TEM mode the current flow should be along the axis of the waveguide, which creates rotating magnetic fields which are normal to the electric fields generated due to the (moving) charges in the current carrying conductor.

Since such a current source is absent and waveguide being a single conductor configuration, TEM mode cannot exist inside a waveguide. Also it is evident from the above explanation that for TEM mode to exist, presence of at least two conductors is compulsory.