The output signal voltage relative to the input signal voltage
The output signal current relative to the input signal current
The output signal power relative to the input signal power
All these choices are correct✓الإجابة الصحيحة
الشرح
Gain is simply the ratio of output to input, and it can be stated for voltage, for current, or for power depending on what matters in the circuit, so all three descriptions are correct. This is why a gain figure should always name which quantity it refers to.