The correct answer is A. Input impedance is equal to output impedance.
Power gain in decibels is equal to voltage gain in decibels only when the input impedance is equal to the output impedance. This is because the power gain is the ratio of the output power to the input power, and the voltage gain is the ratio of the output voltage to the input voltage. When the input impedance is equal to the output impedance, the output power is equal to the input power, and the voltage gain is equal to the power gain.
Option B is incorrect because the output impedance can be any value, and the power gain will still be equal to the voltage gain when the input impedance is equal to the output impedance.
Option C is incorrect because the power gain can be equal to the voltage gain when the input impedance is equal to the output impedance.
Option D is incorrect because the input impedance can be any value, and the power gain will still be equal to the voltage gain when the input impedance is equal to the output impedance.