Which of the following frequencies will be suitable for beyond the horizon communication using sky waves?

Which of the following frequencies will be suitable for beyond-the-Zorizon communication using sky waves ?

Which of the following frequencies will be suitable for beyond-the-horizon communication using the sky waves?

Among the follwing frequencies one will be suitable for beyond-the horizon communication using sky waves is

The frequency suitable for beyond - the - horizon communication using sky waves is

Explain why sky waves ate suitable for long range communication.

Assertion : Electromagentic waves with frequencies more than the cirtical frequency of ionosphere cannot be used for communication using sky wave propagation. Reason : The refractive index of the ionosphere becomes very high for frequencies higher than the critical frequency .