Which factor contributes to a baby's risk of developing rickets?

The correct choice highlights the importance of maintaining adequate levels of vitamin D and phosphorus in a baby's diet, as these nutrients are essential for bone health. Rickets is a condition that results from the softening and weakening of bones in children, primarily due to a deficiency of vitamin D, calcium, or phosphorus.

Vitamin D plays a critical role in calcium absorption from the diet, and phosphorus is vital for bone mineralization. If a baby does not receive enough of these nutrients, it can lead to developmental issues and the risk of rickets increases significantly. This condition is characterized by weakened bones, which can result in deformities and growth problems.

In contrast, factors such as poor dietary choices in adulthood, excessive exercise, or having a healthy appetite do not directly contribute to a baby's risk of developing rickets. While adult dietary habits may influence overall nutrient availability in a household, they do not apply to a baby's immediate risk. Excessive exercise is less of a concern in infants and toddlers, who typically engage in natural play rather than structured exercise routines. A healthy appetite generally indicates that a baby is consuming adequate nutrition, which would typically reduce rather than increase the risk of nutrient deficiencies associated with rickets.