In digital communication system, if both power and bandwidth are limited, then which mechanism/choice is preferred?

The correct answer is: A. Power efficient modulation.

Power efficient modulation is a technique used to transmit data over a communication channel with limited power. It does this by reducing the amount of power required to transmit each bit of data. This can be done by using a modulation scheme that is more efficient at using the available power, or by using a lower signal-to-noise ratio.

Bandwidth efficient modulation is a technique used to transmit data over a communication channel with limited bandwidth. It does this by reducing the amount of bandwidth required to transmit each bit of data. This can be done by using a modulation scheme that is more efficient at using the available bandwidth, or by using a lower symbol rate.

Error control coding is a technique used to detect and correct errors that occur during data transmission. It does this by adding redundant information to the data stream, which can then be used to detect and correct errors.

Trellis coded modulation is a type of modulation that combines power efficient modulation and error control coding. It does this by using a trellis diagram to represent the possible states of the transmitted signal. This allows the receiver to decode the signal even in the presence of errors.

In a digital communication system, if both power and bandwidth are limited, then power efficient modulation is preferred. This is because power efficient modulation can reduce the amount of power required to transmit each bit of data, which can be a significant advantage in systems with limited power. Bandwidth efficient modulation and error control coding can also be used to improve the performance of a digital communication system, but they are not as important as power efficient modulation when both power and bandwidth are limited.