Final answer:
Vitamin D is the vitamin that helps regulate blood calcium levels by enhancing intestinal calcium absorption and aiding the action of PTH. Vitamin C is the water-soluble vitamin among the choices given.
Step-by-step explanation:
The vitamin that helps regulate the blood calcium level is Vitamin D. This is because Vitamin D enhances the absorption of calcium from the intestine and thus maintains normal calcium concentration in the blood.
Parathyroid hormone (PTH), in the presence of low blood calcium levels, stimulates the activation of Vitamin D in the kidneys, which then facilitates the absorption of calcium in the intestines.
Additionally, PTH increases the reabsorption of calcium by the kidneys and stimulates osteoclasts in the bones to release more calcium into the bloodstream. Therefore, the correct answer to which vitamin helps in regulating blood calcium levels is Vitamin D (C).
As for which of the following vitamins is water-soluble, the correct answer is Vitamin C (d). Vitamin C is known to be water-soluble, unlike vitamins A, E, and K, which are fat-soluble.