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The situation described involves a 71-year-old man experiencing worsening vision at night, which is a classic symptom associated with a deficiency in vitamin A. Vitamin A plays a crucial role in maintaining healthy vision, particularly in low-light conditions. It is essential for the production of rhodopsin, a protein in the retina that is necessary for night vision. A deficiency in vitamin A can lead to a condition called night blindness, where the individual struggles to see in dim light or darkness.

In contrast, while the other vitamins mentioned have important roles in the body's functions—niacin is related to energy metabolism and skin health, thiamine is essential for carbohydrate metabolism and nervous system function, and vitamin C is necessary for collagen synthesis and immune function—they do not specifically relate to the visual difficulties described in the scenario. Therefore, recognizing the link between vitamin A deficiency and night vision problems clearly supports the conclusion that the correct answer is vitamin A.