Final answer:
The mature stage of a thunderstorm begins with the formation of the anvil top.
Step-by-step explanation:
In the context of a thunderstorm, the mature stage begins with the formation of the anvil top. This occurs when the updraft reaches the tropopause and spreads out horizontally, creating the characteristic anvil shape at the top of the storm. It is during this stage that the storm is fully developed and can produce heavy precipitation, strong winds, and lightning.