Power amplifiers are electronic devices used to increase the power of a signal. They are commonly used in audio systems, RF transmitters, and other applications where high power output is required. A power amplifier directly amplifies the power of the signal, which is the product of voltage and current. Therefore, it can be said that a power amplifier amplifies both the voltage and current of the signal, ultimately resulting in an increase in power.

Let's discuss each option in detail to understand why the correct answer is option 'D':

a) Voltage of the signal:
A power amplifier does amplify the voltage of the input signal. It takes a low voltage signal and increases its amplitude to a higher voltage level. However, amplifying the voltage alone does not necessarily result in an increase in power. Power is the product of voltage and current, so amplifying one without the other will not increase the overall power of the signal.

b) Current of the signal:
Similar to voltage, a power amplifier also amplifies the current of the input signal. It provides the necessary current to drive the load and deliver the desired power output. However, amplifying the current alone without amplifying the voltage will not result in an increase in power. Therefore, amplifying the current alone is not sufficient for power amplification.

c) Power of the signal:
A power amplifier directly amplifies the power of the signal. As mentioned earlier, power is the product of voltage and current. By increasing both the voltage and current of the input signal, the power amplifier effectively increases the overall power output. This is the main function of a power amplifier and the reason why it is used in applications that require high power delivery.

d) All of the mentioned:
The correct answer is option 'D' because a power amplifier directly amplifies both the voltage and current of the signal, which in turn amplifies the power. By increasing both the voltage and current simultaneously, a power amplifier can deliver a higher power output compared to the input signal.

In conclusion, a power amplifier directly amplifies the power of the signal by amplifying both the voltage and current. This is achieved by increasing the amplitude of the input signal, resulting in a higher power output.