Question

At which of the following frequency/frequencies the communication will not be reliable for a receiver situated beyond the horizon:
(a) 10 kHz
(b) 10 MHz
(c) 1 GHz
(d) 1000 GHz

Answer 1

(b) is correct. Here (c) and (d) frequencies have high penetration power so the earth will absorb them. Radiation (a) of 10 kHz will suffer from the problem of the size of the antenna.