Which fuel/air mixture will produce the highest engine temperature?

The fuel/air mixture that will produce the highest engine temperature is a mixture leaner than a manual lean mixture of .060. Operating with a lean mixture increases the combustion temperature because there is less fuel relative to the amount of air. This causes a more complete combustion of the fuel, often resulting in higher temperatures due to the increased efficiency in the burn process.

In the context of engine performance, a very lean mixture means there's greater oxygen available in the combustion chamber, leading to higher combustion efficiency. This can cause hotter temperatures as the fuel burns more completely. While a mixture that is excessively lean can cause overheating and potential engine damage, within the parameters of the question, moving towards a leaner mixture maximizes combustion efficiency and thus leads to higher temperatures.

The other options present mixtures which are not as lean, meaning they will not produce engine temperatures as high as a mixture leaner than a manual lean mixture of .060.