Parathyroid hormone (PTH) .
a. stimulates kidneys to excrete calcium.
b. tends to inhibit osteoclasts.
c. increases calcium excretion by the intestine.
d. decreases blood calcium levels.
e. allows the kidneys to retain calcium.

The Parathyroid hormone PTH regulates the calcium level by its regulatory effects on bone, kidney and intestine. ALL OPTIONS are correct.

Explanation:

Parathyroid hormone PTH increases the blood calcium level by breaking down the bone and resulting to the release calcium. Parathyroid hormone raises the ability of the body to absorb calcium from foods.

It regulates osteclast proliferation, it regulates the calcium levels in the kidney, bone and intestine.

it is a limiting nutrient limited by a single nutrient in the ecosystem, & the single goes very slow.