We do not transmit low frequency signal to long distances because
(a) The size of the antenna should be comparable to signal wavelength which is unreal solution for a signal of longer wavelength.
(b) Effective power radiated by a long wavelength baseband signal would be high.
(c) We want to avoid mixing up signals transmitted by different transmitter simultaneously.
(d) Low frequency signal can be sent to long distances by superimposing with a high frequency wave as well.
Therefore, the most suitable option will be :
Answer: (C) (a), (c) and (d) are true only
(a) True: an antenna needs a size of order $\lambda/4$, impractically large for long wavelengths.
(b) False: radiated power varies as $(l/\lambda)^2$, so a long-wavelength signal radiates very little power.
(c) True: modulating different signals onto different carriers keeps them from mixing.
(d) True: this is modulation, where the signal rides on a high-frequency carrier.
So (a), (c) and (d) are true.
Solution by Sreeraj P, M.Sc Physics