What is the criteria for a special observation regarding thunderstorms?

The correct answer reflects that a special observation is warranted at the beginning of a new thunderstorm during an ongoing report. This is because timely reporting of thunderstorm activity is crucial for safe aviation operations. When a new thunderstorm starts while a report is already in effect, it can introduce changes in weather conditions that could affect flight safety. Consequently, pilots and air traffic controllers need to be aware of this new information immediately.

Other choices do not meet the specific criteria for issuing a special observation. The first option, which suggests reporting only when a thunderstorm clears, does not provide the necessary real-time information that is essential for safety during the thunderstorm's active period. The choice regarding any nearby thunderstorm activity is too vague and does not specify anything actionable or immediate that pertains to ongoing reporting conditions. Lastly, the option related to changes in cloud height due to thunderstorms does not directly relate to the initiation of new thunderstorm activity, which is the main focus for issuing a special observation.