Correct option is (a) 1
The best I can explain: Distance between transmitter and receiver is \(d=d_t+d_r=\sqrt{2r_e h_t}+\sqrt{2r_e h_r} \)
And re=k*6370km, k is the effective radius factor.
Expression for radio horizon in km is \(d = 3.56(\sqrt{h_t}+\sqrt{h_r}),\) LOS depends on k also so d = \(4.12(\sqrt{h_t}+\sqrt{h_r}).\) At k=1 both will be equal.