Final answer:
A decrease in phosphorus levels leads to an increase in calcium levels due to their reciprocal relationship and homeostasis in the body. The option (A) is correct.
Step-by-step explanation:
A decrease in phosphorus levels will increase calcium levels. This happens because phosphorus and calcium have a reciprocal relationship in the body. High levels of inorganic phosphate can lead to a depression of the solubility of the calcium of bone crystals, which shifts the equilibrium towards bone and decreases serum calcium.
Therefore, when phosphorus levels are lowered, there is less opposition to the effects of the parathyroid hormones, making their activity to increase calcium levels more apparent. This illustrates the importance of maintaining homeostasis within the body. Therefore, option (A) is correct.