An internal supercharger is one which:

An internal supercharger is a device that increases the engine's power output by forcing more air into the combustion chamber. This results in a denser air-fuel mixture that can enhance combustion efficiency and performance. The correct answer indicates that the supercharger compresses the mixture, referring specifically to the air-fuel mixture that is essential for combustion.

When the supercharger compresses the mixture, it ensures that more air and fuel are delivered to the engine cylinders, allowing for a more powerful combustion process. This setup is vital for maintaining optimal engine performance, particularly in high-demand situations such as racing or heavy acceleration.

The other options, although related to the topic of supercharging, do not accurately describe the function of an internal supercharger. For example, a supercharger driven by exhaust gases would pertain more to a turbocharger, which operates differently compared to a supercharger that mechanically compresses the air-fuel mixture. Therefore, the focus on compressing the mixture highlights the primary operational role of the internal supercharger in enhancing engine performance.