What indicates the mature stage of a thunderstorm?

The mature stage of a thunderstorm is primarily indicated by the onset of precipitation at the Earth's surface. At this stage, the thunderstorm has developed sufficient organization, with both strong updrafts and downdrafts, leading to the formation of rain.

During the mature phase, the storm reaches its peak intensity, characterized by heavy rain, thunder, and sometimes hail. The rain reaching the surface signifies that the storm is not only producing significant condensation but also that the airflow dynamics are well established. This contrasts with the earlier stages when updrafts may be present but without any rainfall reaching the ground.

The other options fail to capture the essential characteristics of the mature stage effectively. For example, strong updrafts alone do not indicate maturity, as they can occur in the developing stage as well. Extremely high temperatures at the cloud base could occur without necessarily indicating a mature storm. Additionally, the "quiet phase prior to dissipating" pertains to a later stage of the storm lifecycle rather than maturity. Thus, the initiation of rain at the surface is a clear and definitive marker of a thunderstorm having reached its mature phase.