In an internal combustion engine, detonation is caused by:

Detonation in an internal combustion engine, often referred to as "knocking," occurs when there is an uncontrolled explosion of the air-fuel mixture in the combustion chamber. The phenomenon is primarily driven by excessive combustion temperatures and pressures that reach a point where the mixture ignites prematurely, leading to a rapid and violent release of energy.

When the temperatures and pressures within the cylinder exceed normal operating conditions, the air-fuel mixture can ignite before the spark plug fires, or it can burn at an exceedingly fast rate after ignition. This results in a shock wave that produces the characteristic knocking sound and can cause engine damage over time. Therefore, maintaining appropriate temperature and pressure levels is crucial to preventing detonation and ensuring smooth engine operation.

The other options relate to various engine conditions, but they do not directly lead to the specific situation of detonation. Using an incorrect oil grade can affect engine lubrication but doesn't typically cause detonation. Low cylinder temperatures and pressures would more likely lead to incomplete combustion rather than detonation, while high R.P.M.s with low manifold pressure might impact engine performance but not necessarily induce detonation under normal circumstances.