Which situation would likely cause detonation in an engine?

A high compression ratio in an engine significantly increases the likelihood of detonation, which is the premature combustion of the air-fuel mixture. When the compression ratio is elevated, the air-fuel mixture is compressed to a greater extent, resulting in higher temperatures and pressures before ignition occurs. If the temperature and pressure exceed the fuel's octane rating threshold, the fuel can ignite spontaneously instead of burning smoothly in a controlled manner. This leads to a rapid increase in pressure that can be harmful to engine components and is characterized by a knocking or pinging sound.

In contrast, the other situations identified do not contribute to detonation in a similar manner. High oil viscosity might affect how well the engine lubricates and could affect performance, but it does not directly impact the combustion characteristics of the air-fuel mixture. A low engine temperature can lead to incomplete combustion and may actually prevent detonation by ensuring the fuel-air mix burns more slowly. Lastly, a rich air/fuel ratio (more fuel than air in the mixture) typically decreases combustion temperatures and can reduce the chances of detonation, as there is more fuel available to absorb heat and lower peak combustion temperatures.