Explanation

(d) Given, the frequency of radio signal
$v = 20\ MHz = 20 \times 10^{6}\ Hz$
The wavelength of the antenna is
$\lambda = \frac{c}{v}$
$\lambda = \frac{3 \times 10^{8}}{20 \times 10^{6}}\ or\ \lambda = 15\ m$
$\therefore$ For efficient radiation and reception, the height of transmitting and receiving antennas should be comparable to a quarter wavelength of the frequency used.
$\because$ The minimum length of antenna
$= \frac{\lambda}{4} = \frac{15}{4} = 3.75\ m$