Which of the following substances is critical for bone health?

Calcium plays a crucial role in maintaining bone health due to its function as a primary structural component of bones. The human body requires calcium to build and maintain strong bones and teeth; approximately 99% of the body's calcium is stored in these structures. Without adequate calcium, the body cannot sustain proper bone density and strength, which can lead to conditions such as osteoporosis, making bones more fragile and susceptible to fractures.

Although carbohydrates, proteins, and vitamins also contribute to overall health and nutrition, their roles in relation to bone health are not as direct as that of calcium. While proteins help in building muscle and can support bone strength indirectly through muscle function and maintenance, and vitamins can aid in the absorption and utilization of calcium (especially vitamin D), they do not provide the same foundational role that calcium does in bone structure. Therefore, calcium is essential and more critical for the specific aspect of bone health than the other substances listed.