The correct answer is D) Rickets.
Rickets is a condition that occurs due to a deficiency of vitamin D, calcium, or phosphate in the body. It mainly affects children and can cause weak and soft bones, leading to skeletal deformities.

Vitamin D is essential for the absorption of calcium and phosphate from the intestine. It is also important for maintaining adequate levels of calcium and phosphate in the blood. When there is a deficiency of vitamin D, the body cannot properly absorb and utilize calcium and phosphate, which are crucial for bone health.

Here is a detailed explanation of why rickets is caused by a deficiency of vitamin D:

1. Importance of vitamin D in bone health:
Vitamin D plays a vital role in bone growth and development. It helps the body absorb calcium and phosphate, which are necessary for the formation and mineralization of bones. Without sufficient levels of vitamin D, the body cannot properly utilize these minerals, resulting in weak and soft bones.

2. Effects of vitamin D deficiency on bone health:
When there is a deficiency of vitamin D, the body is unable to absorb enough calcium and phosphate from the diet. As a result, the blood levels of these minerals decrease, leading to a condition called hypocalcemia and hypophosphatemia. Low levels of calcium and phosphate in the blood can cause the bones to become weak and soft.

3. Symptoms of rickets:
Rickets can present with various symptoms, including:

- Delayed growth and development
- Bowing of the legs, also known as genu varum
- Bowing of the spine, leading to a condition called kyphosis or "hunchback"
- Enlarged wrists and ankles
- Muscle weakness and pain
- Dental problems, such as delayed tooth eruption and weak tooth enamel

4. Prevention and treatment of rickets:
Rickets can be prevented and treated by ensuring an adequate intake of vitamin D, calcium, and phosphate. Sunlight exposure is an excellent source of vitamin D, as the body can produce it when the skin is exposed to sunlight. Additionally, vitamin D-rich foods, such as fatty fish, fortified dairy products, and egg yolks, can help maintain sufficient vitamin D levels. In severe cases, vitamin D and mineral supplements may be prescribed by a healthcare professional.

In conclusion, a deficiency of vitamin D can lead to rickets, a condition characterized by weak and soft bones in children. Adequate levels of vitamin D, calcium, and phosphate are essential for proper bone growth and development.