Which weather event would require a special observation?

A special observation is initiated when significant changes occur in weather conditions that may impact aviation safety. In the context of the question, the correct choice is indicative of conditions that warrant immediate attention due to their potential hazard.

The ending of light freezing rain is critical because it presents specific risks such as the formation of ice on airport runways, taxiways, and aircraft surfaces. This can create hazardous conditions for takeoff and landing operations, necessitating an update to ensure pilots and aviation personnel are informed of the latest conditions that could impact safety.

Although the other events mentioned, such as heavy snowfall, the ending of heavy drizzle, and changes in thunderstorm intensity are indeed noteworthy, they do not trigger the need for special observations as immediately as the conclusion of light freezing rain. The primary concern with freezing rain is its ability to create slippery surfaces quickly, prompting a more urgent requirement for updated observations to manage aviation operations effectively.