Stoichiometric Air-Fuel Ratio:
The stoichiometric air-fuel ratio is the ideal ratio of air to fuel that is required for complete combustion to occur. It is the ratio at which all the fuel is burned with the exact amount of oxygen required for complete combustion, resulting in the most efficient combustion process.

Effect of Air-Fuel Ratio on Engine Performance:
The air-fuel ratio has a significant impact on the performance of an internal combustion engine. The combustion process is influenced by the amount of air available for the combustion of fuel. Different air-fuel ratios result in variations in engine power, fuel efficiency, and emissions.

Lean Air-Fuel Mixture:
A lean air-fuel mixture consists of a higher amount of air compared to the stoichiometric ratio. It means there is less fuel available for combustion. This results in incomplete combustion and a decrease in power output. However, it can improve fuel efficiency and reduce emissions.

Rich Air-Fuel Mixture:
A rich air-fuel mixture consists of a lower amount of air compared to the stoichiometric ratio. It means there is more fuel available for combustion. This results in more complete combustion and an increase in power output. However, it can decrease fuel efficiency and increase emissions.

Optimum Air-Fuel Ratio for Maximum Power:
To achieve maximum power output, it is necessary to provide the engine with the optimum air-fuel ratio. In general, a slightly rich air-fuel mixture is preferred for maximum power. This is because a rich mixture helps to cool the combustion chamber, allowing for higher compression ratios and increased power output.

Answer Explanation:
From the given options, the air-fuel ratio regarded for maximum power is option 'C' (12:1 to 14:1). This means that for every 12 to 14 units of air, 1 unit of fuel is used. This slightly rich mixture ensures efficient combustion and provides the necessary cooling effect for maximum power output.

Note:
It is important to mention that the exact air-fuel ratio for maximum power may vary depending on various factors such as engine design, fuel quality, and operating conditions. It is always recommended to refer to the manufacturer's specifications and tuning guidelines for specific engines.