Which of the following can be used to achieve a higher amplifier gain?

Using a more powerful amplifier is an effective way to achieve higher amplifier gain because amplifier gain is fundamentally determined by the design and specifications of the amplifier itself. A more powerful amplifier is typically built with components that can handle larger input signals and provide a greater output signal. This power allows the amplifier to deliver higher voltage and current levels, thereby increasing the gain relative to the input signal.

In contrast, reducing the size of the amplifier casing does not inherently change its gain characteristics; it may even limit the performance if the reduced size affects heat dissipation or component arrangement. Including additional capacitors in the circuit can modify frequency response and stability, but it does not directly increase the amplifier's gain. Decreasing the current through the amplifier would typically lower its output capabilities, further reducing gain rather than increasing it. Therefore, using a more powerful amplifier is the most direct and effective method to achieve a higher amplifier gain.