The correct answer is D. Modulation.
Modulation is the process of encoding information into a carrier wave. This is done by varying one or more properties of the carrier wave, such as its amplitude, frequency, or phase. The modulated signal is then transmitted over a communication channel, such as a radio wave or a fiber optic cable.
The receiver demodulates the signal by extracting the information that was encoded into the carrier wave. This is done by reversing the modulation process.
Modulation is used to improve the performance of communication systems in a number of ways. For example, modulation can be used to increase the range of a communication system by making the signal more resistant to noise and interference. Modulation can also be used to increase the capacity of a communication system by allowing multiple signals to be transmitted over the same channel at the same time.
However, modulation does not improve the received signal quality over small scale times and distance. In fact, modulation can actually degrade the received signal quality if it is not done correctly. This is because modulation introduces additional noise and interference into the signal.
Diversity is a technique that is used to improve the received signal quality by combining multiple copies of the same signal. This is done by using multiple antennas or by using multiple paths to transmit the signal. Diversity can be used to improve the received signal quality over small scale times and distance by reducing the effects of fading and shadowing.
Channel coding is a technique that is used to improve the received signal quality by adding redundancy to the signal. This is done by encoding the data in a way that makes it more resistant to noise and interference. Channel coding can be used to improve the received signal quality over small scale times and distance by reducing the effects of errors and corruption.
Equalization is a technique that is used to improve the received signal quality by compensating for the effects of the communication channel. This is done by adjusting the received signal in a way that makes it more like the original signal. Equalization can be used to improve the received signal quality over small scale times and distance by reducing the effects of distortion and delay.