Low octane will cause?

Low octane fuel is primarily associated with causing spark knock, also known as pre-ignition or knocking. This phenomenon occurs because lower octane fuels have a higher tendency to ignite under pressure and heat in the combustion chamber. When the air-fuel mixture ignites too early, it creates a knocking sound and can lead to engine damage over time if not addressed.

The reason spark knock is the most relevant effect of using low octane fuel is that higher octane fuels are designed to resist knocking, making them ideal for high-compression engines that require precise combustion timing. Running a lower octane fuel in such engines can lead to performance issues, reduced power, and potential long-term damage.

While it can be argued that low octane fuel could contribute to overheating or excessive carbon buildup under certain conditions, these issues are more indirect and not as prominently associated with low octane fuel as spark knock. Therefore, the specific and direct correlation of low octane fuel resulting in spark knock makes it the best choice among the options provided.