Explanation:

When the mixture is lean, it means that the air-fuel ratio is higher, i.e., there is more air compared to fuel in the mixture. This has several implications on the combustion process and the performance of the engine.

1. Efficiency:
When the mixture is lean, the combustion process becomes less efficient. This is because there is not enough fuel available for complete combustion. In a stoichiometric mixture (the ideal air-fuel ratio for complete combustion), all the fuel is burned completely with the available oxygen, resulting in maximum energy release. However, in a lean mixture, some of the fuel remains unburned due to insufficient oxygen. This incomplete combustion leads to lower energy release and, therefore, lower efficiency.

2. Power Output:
The power output of an engine is directly related to the energy released during combustion. As mentioned earlier, a lean mixture results in incomplete combustion and lower energy release. Consequently, the power output of the engine is reduced when the mixture is lean.

3. Maximum Temperature and Pressure:
In a lean mixture, the maximum temperature and pressure during combustion are higher compared to a stoichiometric mixture. This is because the excess air in the mixture tends to increase the combustion temperature. However, it is important to note that although the maximum temperature and pressure may be higher, the overall energy release and power output are still lower due to incomplete combustion.

Therefore, when the mixture is lean:
- The efficiency of the combustion process is reduced.
- The power output of the engine is decreased.
- The maximum temperature and pressure during combustion may be higher, but this does not compensate for the lower energy release and power output.

Hence, the correct answer is option B: Power output is less.