Modulation is used to:
A. reduce the bandwidth
B. To separate the transmission of different users
C. To ensure that intelligence may be transmitted to long distances
D. To allow the uses of practical antenna

Hint:In this question we will learn about modulation and modulation is one of the crucial branches of electronics science that is widely used in communication systems. It takes into account the signal's various fundamental properties in order to transpose it from one location to another.

Complete answer:
Modulation and its various types prevent other signals from interfering with the message signal. It's because a person sending a message signal over the phone won't be able to distinguish between them. As a result, they'll get in each other's way. The mixing of the signals can be avoided by using carrier signals with a high frequency. As a result, modulation guarantees that the signals received by the receiver are flawless.

From this we can say that Modulation is used to separate the transmission of different users.
The sender's baseband signals aren't capable of being transmitted directly. The message signal's strength should be increased to allow it to travel longer distances. Modulation is crucial in this situation. The most important requirement of modulation is to increase the signal's strength without affecting the carrier signal's parameters.

From this we can say that Modulation is used to ensure that intelligence may be transmitted to long distances.Signals with a frequency range of 20 Hz to 20 kHz can only travel a short distance. The length of the antenna used to send the message signal should be a quarter wavelength of the used frequency. As a result, modulation is required to increase the message signal's frequency and strength so that it can reach the receiver.From this we can say that Modulation is used to allow the use of practical antennas.

So, from above, we can conclude that option B, C, D are correct.

Note:Modulation can be defined as the process of conversion of data into waves by adding information to a carrier signal. Such a signal can be an electronic or an optic carrier, but most importantly, it should possess a steady waveform. The amplitude and frequency of a signal with a steady waveform are both constant. Furthermore, one can add information to such a carrier by modulating it by changing its frequency, phase, amplitude, or polarisation in the case of optical signals, as well as changing the spin in the case of electronic or quantum signals.