Final answer:
Plant oils are a rich source of Vitamin E, which is known for its antioxidant properties. It is also essential for protecting other vitamins from oxidation and is prevalent in various plant oils such as sunflower and olive oil.
Step-by-step explanation:
Plant oils are a rich source of Vitamin E, which is chemically known as α-tocopherol. It is a potent antioxidant, protecting cells from oxidative stress by scavenging harmful by-products of metabolism such as free radicals. Vitamin E also aids in protecting other vitamins, like Vitamin A, from oxidation. Certain plant oils such as sunflower, safflower, canola, and olive oil are especially high in Vitamin E.
To answer the student's question, plant oils are a rich source of 4) Vitamin E.
To provide additional context, Vitamin A, D, E, and K are all fat-soluble vitamins, whereas Vitamin C is water-soluble. Given that plant oils contain dietary lipids, they aid in the absorption of fat-soluble vitamins.
As for the student's previous question, which was not to be directly answered but used for reference, the water-soluble vitamin among the options given is d. Vitamin C.