Which factor can cause engine overheating during high power operations?

The correct answer, "too lean a mixture," is significant because when the fuel-to-air ratio is too lean, it means there is insufficient fuel within the mixture to absorb heat generated during combustion. This can result in higher temperatures within the engine cylinders. In high power operations, where the engine is working hard and generating more heat than during lower power operations, this effect is amplified.

Inadequate fuel can lead to combustion that reaches excessively high temperatures, which are detrimental to engine components. The danger of engine overheating is particularly pronounced in conditions where power output is maximized, as the lack of fuel not only leads to a lean mixture but also to less effective cooling.

Other options pose different issues such as rich mixtures which can lead to incomplete combustion or excessive RPMs that might cause mechanical strain, while low manifold pressure might not directly relate to engine temperature in high-power situations. In contrast, a lean mixture is directly linked to the potential for overheating during demanding operations.