Osteoporosis involves which of the following?

Osteoporosis primarily involves the skeletal system, as it is a condition characterized by weak and brittle bones, resulting from a loss of bone density. This condition occurs when the creation of new bone doesn't keep up with the removal of old bone, leading to an increased risk of fractures. Although osteoporosis directly affects the skeletal system, factors such as hormonal changes, diet, and lifestyle can indirectly involve other systems.

In reference to the other options, while the skeletal system is a critical component affected by osteoporosis, the digestive and reproductive systems also play roles in overall bone health. The digestive system is important because it influences nutrient absorption; nutrients like calcium and vitamin D are crucial for bone maintenance. The reproductive system is involved due to hormones that affect bone density, particularly estrogen. Thus, the combination of the skeletal, digestive, and reproductive systems makes the selected answer most reflective of the complex nature of osteoporosis and its contributing factors.