The correct option is (c) sky waves.
Explanation:
Maximum distance covered by space wave communication
Since the distance between transmitter and receiver is 110 km, therefore the given frequency signal cannot communicate through either by ground waves or space wave,
For sky waves, the value of critical frequency is
where, Nmax = Maximum electron density of ionosphere.
As the signal frequency 5 MHz < 9 MHz, so the communication is through sky waves.