What is a key function of vitamin D?

Vitamin D plays a crucial role in the growth and development of bones and teeth, which is why the correct answer is focused on this function. It aids in the absorption of calcium and phosphorus, two essential minerals that are critical for bone health. Without adequate vitamin D, bones can become thin, brittle, or misshapen, leading to conditions such as rickets in children and osteomalacia or osteoporosis in adults.

The other options highlight functions that are not directly associated with vitamin D. For example, while hair growth is influenced by various vitamins and minerals, it is not a primary function of vitamin D. Similarly, blood sugar regulation is more closely linked to hormones such as insulin rather than vitamin D. Finally, water retention is primarily managed by the kidneys and certain hormones, rather than by vitamin D, which does not have a direct role in this process. Therefore, the emphasis on the relationship between vitamin D and bone and teeth health accurately reflects its vital importance in overall skeletal integrity.