**Core Concept**
Calcium homeostasis is a tightly regulated process that maintains the balance of calcium ions within the body. It involves the interplay of multiple organs and hormones to ensure that calcium levels remain within a narrow range, crucial for various physiological processes including muscle contraction, nerve function, and bone health.

**Why the Correct Answer is Right**
The correct answer is **Kidneys**. The kidneys play a crucial role in regulating calcium levels by excreting excess calcium in the urine, but they are not directly involved in the active regulation of calcium homeostasis. The primary organs involved in calcium homeostasis are the parathyroid glands, which secrete parathyroid hormone (PTH), and the bones, which store and release calcium.

**Why Each Wrong Option is Incorrect**
* **Option A:** Liver - The liver plays a role in vitamin D metabolism, which is essential for calcium absorption in the gut. However, it is not directly involved in the regulation of calcium homeostasis.
* **Option B:** Pancreas - The pancreas secretes calcitonin, a hormone that helps regulate calcium levels by inhibiting osteoclast activity and promoting calcium deposition in bones. However, the pancreas is not the primary organ involved in calcium homeostasis.
* **Option C:** Bones - The bones are a critical component of calcium homeostasis, storing and releasing calcium as needed to maintain serum calcium levels.

**Clinical Pearl / High-Yield Fact**
The kidneys are responsible for adjusting calcium excretion in response to changes in serum calcium levels, but they do not actively regulate calcium homeostasis. This is why patients with chronic kidney disease often develop hyperparathyroidism, leading to elevated serum calcium levels.

**Correct Answer: D. Kidneys**