The correct answer is option 2, Vitamin C. Vitamin C, also known as ascorbic acid, is highly susceptible to deterioration when exposed to air. This is because Vitamin C is an unstable compound that can easily undergo oxidation. This oxidation process is accelerated when Vitamin C comes in contact with air, heat, light, and certain metals.

On the other hand, Vitamin A (option 1), Vitamin D (option 3), and Vitamin E (option 4) are relatively stable vitamins that are not as easily affected by exposure to air. Vitamin A is more sensitive to heat and light, while Vitamin D and Vitamin E are more stable and less prone to deterioration compared to Vitamin C.

It is important to note that the deterioration of Vitamin C can occur over time, especially when fruits and vegetables high in Vitamin C are left exposed to air or stored improperly. To preserve the Vitamin C content in food, it is recommended to consume fresh fruits and vegetables promptly, store them in airtight containers, and minimize exposure to air, heat, and light.

As a result, option 2, Vitamin C, is the correct answer to the question.