An audio signal of 15kHz frequency cannot be transmitted over long distances without modulation because
(a) the size of the required antenna would be at least 5 km which is not convenient.
(b) the audio signal can not be transmitted through sky waves.
(c) the size of the required antenna would be at least 20 km, which is not convenient.
(d) effective power transmitted would be very low, if the size of the antenna is less than 5 km.