An electromagnetic wave has energy in the form of
A. variable electric field
B. variable magnetic field
C. both A and B
D. none of the above

Hint: Try to remember the definition of an electromagnetic wave. You may realize that they are formed due to the interaction of electric and magnetic fields. Also, recall the expression for the energy of an electromagnetic wave.

 Complete step by step answer:
Let us first discuss electromagnetic waves. They are created as a result of vibrations between an electric and magnetic field, to make it more clear, they consist of oscillating electric and magnetic fields. The very name of them says about their formation. So, the electric and magnetic fields of an electromagnetic wave are at right angles to each other. Also, the propagation direction is perpendicular to them.

They are known to travel with a constant velocity of $3\times {{10}^{8}}m{{s}^{-1}}$. You may be quite familiar with this velocity. Yes! This is the universal speed of light ‘c’. Recall that light is indeed an electromagnetic wave. Neither electric nor magnetic field can cause deflection to an EM wave but they are capable of diffraction as well as interference. Unlike mechanical waves (string waves, sound waves and water waves) EM waves don't require a medium for propagation. They travel through just anything, be it air water, solid or even vacuum.
It was the Scottish scientist named James Clerk Maxwell who noticed that electric and magnetic fields can couple together to form EM waves. Maxwell also has a set of equations that explains this relationship between electricity and magnetism called ‘Maxwell’s equations’.
Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays form different regions of the electromagnetic spectrum.
Since, we know that EM waves are formed from the combination of both electric and magnetic fields their energy must be in the form of variable electric fields as well as variable magnetic fields. Also, the for an electromagnetic force, energy can be given by,
$\dfrac{1}{2}{{\varepsilon }_{0}}{{E}^{2}}=\dfrac{{{B}^{2}}}{2{{\mu }_{0}}}$

So, the correct answer is “Option C”.

Note:
Higher the frequency, higher is the energy of the given Electromagnetic wave. That is, energy is directly proportional to the frequency but inversely proportional to the wavelength. Gamma rays lie in the higher frequency region of the electromagnetic spectrum hence have higher energy than others. Radio waves have the highest wavelength and lowest frequency and hence, least energy.