Which of the following vitamin helps to absorb calcium in the body and…

Vitamin D is a fat-soluble vitamin whose primary physiological role is regulating calcium and phosphate metabolism: it increases calcium absorption from the small intestine, works with parathyroid hormone to keep blood calcium levels stable, and promotes mineralisation (hardening) of bone tissue.

Among the four vitamins listed, vitamin D is the one whose defining physiological role — increasing intestinal calcium absorption and supporting bone mineralisation — directly matches what the question asks.

Cross-check — contrast with the other vitamins by their own actual roles:

• Vitamin A's well-established roles are in vision (rhodopsin synthesis), maintaining epithelial tissue, and supporting immune function — it plays no direct role in calcium absorption.

• Vitamin C is required for collagen synthesis and acts as an antioxidant — it does not regulate intestinal calcium absorption.

• Vitamin K's defining role is enabling the blood-clotting cascade through clotting-factor synthesis — a function unrelated to intestinal calcium absorption.