When using a vacuum gauge connected to the intake manifold, which statement is true?

The choice indicating that when the throttle is released suddenly from 2500 RPM, the needle will snap to a higher than normal reading for an instant, accurately describes the behavior of a vacuum gauge in this scenario. When the throttle is suddenly closed, there is a momentary increase in manifold vacuum due to the abrupt restriction of airflow. This occurs because the engine is still running at a high speed, creating greater vacuum as air is drawn into the manifold against a closed throttle plate. The needle on the vacuum gauge reflects this momentary spike.

In contrast, the other statements do not accurately reflect normal engine behavior with respect to vacuum readings. For instance, during rapid acceleration, one would expect to see a drop in vacuum rather than a rise, as the throttle opens up and the engine draws in more air, reducing manifold vacuum. Additionally, a "normal" gauge reading at idle is generally higher, typically around 16-22 inches of mercury, varying by engine design and condition. These nuances in the operation of a vacuum gauge are critical for diagnosing engine performance and identifying potential issues.