Modulation is used to (A) reduce the band width used (B) separate the transmission of different areas (C) ensure that information may be transmitted to long distance (D) allow the use of practical antennas

Modulation is the process of varying one or more properties of a carrier wave, such as its amplitude, frequency, or phase, in accordance with the information signal. This is done to transmit the information in the form of a modulated signal.

There are several reasons why modulation is used in communication systems: 1. To reduce the frequency of the signal so that it can be easily transmitted over long distances and to allow the use of practical antennas. 2. To allow multiple users to coexist in the same communication system and send their information signals simultaneously without interference. 3. To improve the signal-to-noise ratio and the overall quality of the communication.

(A) Modulation does not directly reduce the bandwidth used; instead, modulation techniques can be chosen according to requirements on bandwidth and other factors. (B) Modulation indeed helps in separating the transmission of different information signals by assigning them different frequencies or other distinct properties. However, this is not an exclusive purpose of modulation. (C) One key purpose of modulation is to ensure effective transmission over long distances by reducing the frequency of the information signal, which in turn helps to decrease signal attenuation. (D) Modulating the signal allows for the use of practical antennas, as the wavelength of a low-frequency signal can be immense, making antenna design for such frequencies impractical.

Based on the explanation above, the correct option is (C) ensure that information may be transmitted to long distance. Modulation helps improving the ability to transmit information over long distances, and it does influence antenna design, but option (D) is not its main purpose.