Correct option: (4) 4 km
Explanation:
Considering the upper limit of the audio frequency range (i.e. 20 kHz), the wavelength, of the corresponding e.m. wave (of this frequency), would be
The optimum size of the antenna being, of the order of λ/4, we need to have an antenna size of the order of 3.75 km (= 4 km).