Mean Effective Pressure (MEP) is the correct answer for comparing engines of different cylinder dimensions, power, and speed. MEP is a measure of the average pressure exerted on the piston during the power stroke of the engine cycle. It is a key parameter for evaluating the performance of an engine.

Here's a detailed explanation of why MEP is the most appropriate criterion for comparison:

1. Maximum Pressure:
The maximum pressure in the cylinder is an important parameter, but it alone cannot provide a comprehensive analysis of engine performance. It only represents the peak pressure that occurs during the combustion process. Engines with different cylinder dimensions, power, and speed can have different maximum pressures, but this does not necessarily reflect their overall efficiency or power output.

2. Fuel Consumption:
Fuel consumption is another important aspect of engine performance, but it can be influenced by various factors such as engine efficiency, load, speed, and design. It is not directly related to cylinder dimensions, power, or speed. Comparing engines solely based on fuel consumption would not give a complete picture of their performance.

3. Unit Power:
Unit power refers to the power output per unit of engine displacement or cylinder volume. It is a useful measure for comparing engines of similar sizes. However, when comparing engines with different cylinder dimensions, power, and speed, unit power alone may not provide an accurate assessment of their performance. It does not take into account factors such as combustion efficiency, mechanical losses, or the effect of different stroke lengths and bore sizes.

4. Mean Effective Pressure:
MEP represents the average pressure exerted on the piston during the power stroke. It is calculated by dividing the work done during the power stroke by the displacement volume. MEP takes into account factors such as combustion efficiency, cylinder dimensions, and engine speed, making it a more comprehensive parameter for comparing engines of different sizes and power outputs.

MEP is directly related to engine efficiency and power output. Higher MEP values indicate more efficient combustion and better power output. Therefore, comparing engines based on their mean effective pressure provides a more accurate assessment of their performance, especially when considering engines with different cylinder dimensions, power, and speed.