The correct answer to this question is option 'D' - Voltage gain is increased. Let's understand why this is the correct answer.

Bypass Capacitor and Common-Emitter BJT Amplifier:
A bypass capacitor is often used in common-emitter BJT amplifiers to improve the performance of the amplifier. It is connected across the emitter and the resistor in the amplifier circuit.

Effect on Input Impedance:
The bypass capacitor does not have a significant effect on the input impedance of the common-emitter BJT amplifier. The input impedance is mainly determined by the base-emitter junction of the transistor and the biasing resistors. The bypass capacitor does not alter these components, so the input impedance remains relatively unchanged.

Effect on Output Impedance:
The bypass capacitor does not have a significant effect on the output impedance of the common-emitter BJT amplifier. The output impedance is primarily determined by the collector resistor and the collector-emitter junction of the transistor. The bypass capacitor does not alter these components, so the output impedance remains relatively unchanged.

Effect on Voltage Gain:
The bypass capacitor plays a crucial role in improving the voltage gain of the common-emitter BJT amplifier. It achieves this by bypassing the emitter resistor, effectively reducing the negative feedback in the amplifier circuit. This reduction in negative feedback increases the voltage gain of the amplifier.

When the bypass capacitor is connected across the emitter resistor, it provides a low impedance path for the AC signal to bypass the emitter resistor. This reduces the voltage drop across the emitter resistor, resulting in an increase in the effective voltage gain of the amplifier.

The bypass capacitor essentially decouples the emitter resistor from the AC signal, allowing the AC signal to be amplified without significant attenuation. This increases the overall voltage gain of the amplifier circuit.

In conclusion, when a bypass capacitor is connected across the emitter and resistance of a common-emitter BJT amplifier, the voltage gain is increased. This occurs because the bypass capacitor reduces the negative feedback in the amplifier circuit, allowing for a higher voltage gain.