During which stage does rain begin to fall in a thunderstorm?

The mature stage of a thunderstorm is characterized by the development and precipitation response of the storm. It is during this stage that the storm reaches its maximum intensity, which includes the formation of heavy rain, thunder, and lightning. The upward movement of air leads to the condensation of water vapor, and eventually, this condensation results in raindrops that begin to fall to the ground.

As the storm matures, processes such as the downdraft and updraft interactions become significant. The downdrafts help to pull the accumulated moisture down from the cloud, leading to the onset of rainfall. This stage also features the development of the anvil-shaped cloud top, which is indicative of a strong updraft. Hence, it is the mature stage where one would expect to see rain beginning to fall from a thunderstorm, making this option the correct answer.