Stoichiometric Air/Fuel Ratio
The stoichiometric air/fuel ratio is the ideal ratio of air to fuel that is required for complete combustion of the fuel. For petrol, the stoichiometric air/fuel ratio is 15:1, which means that 15 parts of air are required to completely burn 1 part of fuel.

Maximum Power Air/Fuel Ratio
To achieve maximum power output from an internal combustion engine, the air/fuel mixture needs to be adjusted. The optimal air/fuel ratio for maximum power varies depending on the specific engine design and operating conditions. However, it is generally observed that an air/fuel ratio slightly richer than the stoichiometric ratio is required to achieve maximum power.

Explanation of Option D (9:1 - 11:1)
Option D states that the air/fuel ratio required for maximum power is between 9:1 and 11:1. This means that for every 9 to 11 parts of air, 1 part of fuel is required.

The reason for this is that a slightly rich mixture provides more fuel for combustion, resulting in higher energy release and increased power output. When the air/fuel mixture is leaner (higher air-to-fuel ratio), the combustion process is incomplete, leading to reduced power output.

Other Options:
- Option A (16:1 - 18:1) suggests a leaner air/fuel ratio compared to the stoichiometric ratio. This would result in incomplete combustion and reduced power output.
- Option B (15:1) represents the stoichiometric air/fuel ratio, which is not optimal for maximum power.
- Option C (12:1 - 18:1) includes a wide range of air/fuel ratios, some of which are leaner than the stoichiometric ratio. These ratios would not provide the maximum power output.

Therefore, option D (9:1 - 11:1) is the correct answer as it represents the air/fuel ratio range required for maximum power in a petrol engine.