Lack of this vitamin can lead to impaired immune function and vision problems?

Vitamin A is essential for maintaining proper immune function and vision. It plays a critical role in the health of the eyes, particularly in the formation of rhodopsin, a pigment found in the retina that is necessary for low-light vision. A deficiency in Vitamin A can lead to issues such as night blindness and more severe eye problems. Additionally, Vitamin A contributes to the proper functioning of the immune system by supporting the epithelial tissues and mucous membranes that act as barriers to infection.

In contrast, while Vitamin D is important for bone health and immune function, and Vitamin E acts as an antioxidant, neither is specifically known for the kind of vision problems associated with Vitamin A deficiency. Vitamin K is primarily involved in blood clotting. Thus, the unique role of Vitamin A in vision and immunity makes it the correct answer.