Option 1: Riboflavin is a vitamin that is sensitive to light and oxygen rather than heat. It can be partially destroyed by heat, but it is not the most readily destroyed by heat.

Option 2: Ascorbic acid, also known as vitamin C, is the correct answer. It is highly sensitive to heat and can be easily destroyed during cooking or processing. It is recommended to consume fruits and vegetables raw or lightly cooked to retain its vitamin C content.

Option 3: Tocopherol, also known as vitamin E, is relatively stable and not as easily destroyed by heat compared to vitamin C.

Option 4: Thiamine, also known as vitamin B1, is stable under normal cooking conditions and is not significantly destroyed by heat.

In conclusion, the correct answer is option 2, ascorbic acid, which is the vitamin most readily destroyed by heat. Alert - correct answer should be option 1, Riboflavin.