Answer: D. 4 db

The power gain of an amplifier is defined as the ratio of the output power to the input power, expressed in decibels (dB).

In this case, the input power is 0.1 W, and the output power is 15 W.

Thus, the power gain of the amplifier is 15/0.1 = 150.

When this ratio is expressed in decibels, the power gain is 10 log10 (150) = 4 dB.

Thus, the correct answer is option D. 4 dB.

The decibel (dB) is a unit of measurement used to express the ratio of two values of a physical quantity, often power or intensity. It is used to express the level of amplification or attenuation of a signal, and is commonly used in audio and radio engineering.