Final answer:
The transition region is the thin layer above the chromosphere of the Sun where temperatures rise sharply from about 10,000 K to nearly a million degrees, leading into the corona.
Step-by-step explanation:
The region of rapid temperature increases that separates the Sun's chromosphere from the corona is the transition region. Above the chromosphere, the temperature rises sharply from about 10,000 K to nearly a million degrees, marking the transition to the corona, which is the outermost part of the Sun's atmosphere. The transition region is very thin, possibly just a few tens of kilometers thick.