The author here is concluding that low-protein diets with fruits and vegetables and weight-bearing exercise cause osteoporosis prevention rather than high-calcium diets.
A. The final sentence of the stimulus states “Weight-bearing exercise… is also essential” to preventing osteoporosis. Answer choice A supports this by showing that Astronauts experience a decrease in bone density despite the vigorous physical activity. If physical activity, in general, was sufficient to prevent osteoporosis, you would expect these astronauts to have thick bones in light of their vigorous physical activity. The fact that they do not have thick bones is strong evidence that the "weight-bearing” part of the exercise is a contributing factor to the resulting increase in bone health. One common way of strengthening a causal statement is by showing that without the cause (weight-bearing exercise in this case) the effect is not present (thick bones). In this way, this answer choice strengthens the author’s view that weight-bearing exercise, amongst other factors, helps in preventing osteoporosis.
C. Another common way of strengthening a causal statement is by showing that when the cause occurs, the effect follows. This answer choice shows that when a low-protein diet is followed, the result is a low incidence of osteoporosis.
