Vitamin D is the correct answer. Vitamin D plays a crucial role in the absorption of calcium by the body. It aids in the absorption of calcium from the intestines into the bloodstream, which is necessary for maintaining strong and healthy bones. Without sufficient vitamin D, the body cannot effectively absorb the calcium consumed through diet or supplements.

Vitamin A (Option 1) is not directly involved in the absorption of calcium. However, it is essential for proper bone growth and development.

Vitamin B (Option 3) and Vitamin C (Option 4) are not directly involved in the absorption of calcium. Vitamin B is important for overall health and energy production, while Vitamin C is known for its role in collagen synthesis and immune function.

Therefore, the correct answer is Vitamin D (Option 2).