Which of the following is a sign of an overheating engine?

Steam from the engine is indeed a clear indicator of an overheating situation. When an engine becomes excessively hot, the coolant may exceed its boiling point, resulting in the formation of steam. This can occur due to a failure in the cooling system, insufficient coolant, or other malfunctions. The presence of steam is a visible sign that the engine is not only overheating but may also be at risk of significant damage if not addressed promptly.

In contrast, increased fuel efficiency is typically associated with optimal engine performance and does not correlate with overheating. Reduced engine noise might suggest that an engine is functioning smoothly or that it has stalled, not necessarily that it is overheating. Lastly, a stable coolant temperature is a sign of a well-functioning cooling system, indicating that the engine is operating within its safe temperature range. In summary, steam from the engine is the most direct and alarming sign of overheating that should prompt immediate investigation and action.