. . . . . . . . is the thermo-sensitive vitamin.

Vitamin-C is the thermo-sensitive vitamin.
Vitamin-C is a water-soluble and temperature-sensitive vitamin, so is easily degraded during cooking and elevated temperatures and long cooking times have been found to cause particularly severe losses of Vitamin-C. It can also leach out of vegetables when they're immersed in hot water.
It functions as a powerful antioxidant in the body and plays important roles in immune function, neurotransmitter production, collagen synthesis.