Electromagnetic waves with frequencies greater than the critical frequency of lonosphere cannot be used for communication using sky wave propagation because

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion.
B. if both assertion and reason are true but reason is not the correct expalanation of the assertion
C. if assertion is true , but reason is false
D. both assertion and reason are false statements.

Explanation

If frequencies of the waves are less thaat of the critical frequency. They will be reflected back as the ionosphere has refractive index less than free space. This is the principle, of sky wave porpagation.
Theforem the assertion is wrong.
The statement given in the reason that the refactive index. becomes high is corect. But the reason given is not true for all assertion.
Assertion is wrong. Reason-statement is correct. But not hte reason. We have to put (e) as this is a rare case where teh assertion itself is wrong.