Electromagnetic waves with frequencies greater than the critical frequency of ionosphere cannot be used for communication using sky wave propagation, because
(A) the refractive index of the ionosphere becomes very high for $f > {f_c}$
(B) the refractive index of the ionosphere becomes very low for $f > {f_c}$
(C) the refractive index of the ionosphere becomes very high for $f < {f_c}$
(D) None of the above

Hint: Here we will use the definition and the concept of sky wave propagation to solve the question. It refers to the propagation of radio waves which are reflected or refracted back towards the planet Earth from the ionosphere which is an electrically charged layer belonging to the upper atmosphere.

Complete answer:
Sky wave propagation is not limited by the curvature of the Earth, so it can be used for the purpose of communication beyond the horizon, at intercontinental distances. It is frequently used in the shortwave frequency.

The electromagnetic waves which have greater frequencies than the critical frequency of the ionosphere cannot be used for communication using sky wave propagation, as they pass through the ionosphere into the outer space. The process of internal reflection will not take place for higher frequency as the refractive index of the ionosphere becomes very high for$f > {f_c}$.

Therefore, the correct answer is option A.

Note: Whenever high frequency waves enter the ionosphere in an oblique way, they are scattered back from the ionized layer in the form of scatter waves. Another important thing to be noted is that the sky wave propagation takes place in the waveguide formed by the ionosphere and the ground, in which both serve as reflectors.