What vitamin is crucial for the absorption of dietary calcium and phosphorus?

Vitamin D plays a pivotal role in the absorption of dietary calcium and phosphorus, which are essential minerals for maintaining healthy bones and overall skeletal development in horses and other animals. When vitamin D is present, it enhances the intestinal absorption of these minerals from the diet. This process is crucial because, without adequate vitamin D, even if calcium and phosphorus are available in the diet, the horse may not be able to absorb sufficient amounts, potentially leading to deficiencies.

Moreover, vitamin D also helps maintain proper levels of calcium and phosphorus in the blood, ensuring that the body can utilize these minerals effectively for critical functions such as bone formation and muscle contraction. Its significance in regulating calcium homeostasis underscores why it is considered essential for equine health, particularly regarding skeletal integrity.

The other vitamins listed—K, A, and B12—while important for various bodily functions, do not have the same direct role in enhancing the absorption of calcium and phosphorus as vitamin D does. Hence, option A is the correct choice for this question.