The Miller effect is a phenomenon that occurs in electronic circuits, specifically in amplifiers, and it affects the high-frequency response of the circuit. In the context of a Common Emitter amplifier, the Miller effect results in a decrease in the high-frequency cutoff frequency.

Explanation:
1. Common Emitter Amplifier:
A Common Emitter amplifier is a basic transistor amplifier configuration where the emitter of the transistor is common to both the input and output signals. It provides voltage amplification and is commonly used in audio applications.

2. Capacitive Coupling:
In a Common Emitter amplifier, the input and output are usually coupled through capacitors. These capacitors provide AC coupling, allowing the amplification of the AC input signals while blocking any DC bias.

3. Miller Effect:
The Miller effect is a result of the interaction between the input and output capacitances of the transistor. It occurs when the input and output capacitors are effectively multiplied by a factor known as the Miller capacitance.

4. Miller Capacitance:
The Miller capacitance (Cm) is given by the formula Cm = C × (1 + Av), where C is the capacitance value and Av is the voltage gain of the amplifier. The voltage gain is negative in a Common Emitter configuration, resulting in a positive Miller capacitance.

5. High-Frequency Response:
The Miller effect significantly increases the effective value of the output capacitance, which reduces the high-frequency response of the amplifier. This increased capacitance causes a decrease in the effective impedance at high frequencies, resulting in a decrease in the high-frequency cutoff frequency.

6. Decrease in High-Frequency Cutoff Frequency:
The high-frequency cutoff frequency is the frequency at which the gain of the amplifier starts to decrease. Due to the Miller effect, the increased Miller capacitance reduces the effective impedance at high frequencies, causing the gain to start decreasing at a lower frequency than expected. This results in a decrease in the high-frequency cutoff frequency of the Common Emitter amplifier.

Therefore, the correct answer is option 'D' - a decrease in the high-frequency cutoff frequency. The Miller effect reduces the high-frequency response of the amplifier, leading to a decrease in the high-frequency cutoff frequency.