Which of the following issues is likely to make an engine difficult to start?

An engine may struggle to start for several reasons, but the most notable issue among the choices that can lead to starting difficulties is having no oil in the crankcase. Oil is essential for proper lubrication of engine components, and without it, the engine cannot function correctly. The lack of oil leads to increased friction between moving parts, which can cause the engine to seize or prevent it from turning over altogether. This condition requires immediate attention, as running an engine without oil can lead to severe damage.

While low fuel pressure, dirty spark plugs, and low engine temperature can also contribute to starting problems, they do not have the same immediate, catastrophic effect that the absence of oil does. Low fuel pressure might prevent the engine from receiving adequate fuel, dirty spark plugs can hinder proper ignition, and low engine temperatures might necessitate longer cranking times, especially in cold starts. However, these issues generally allow for some functionality, whereas no oil in the crankcase can lead to complete engine failure and a much more serious situation.