## Core Concept
The question tests the understanding of management strategies for hypercalcemia, specifically focusing on the rapid lowering of calcium levels. Hypercalcemia can be life-threatening and requires immediate treatment. Various agents can lower calcium levels, but they work through different mechanisms and have different onset times.

## Why the Correct Answer is Right
The correct answer, **Calcitonin**, acts rapidly to lower serum calcium levels. It works by inhibiting osteoclast activity in bones, reducing calcium release into the bloodstream, and increasing renal excretion of calcium. Calcitonin has a quick onset of action, often within hours, making it suitable for emergency situations where rapid calcium lowering is required.

## Why Each Wrong Option is Incorrect
- **Option A: Bisphosphonates**. While bisphosphonates are potent in lowering calcium levels, their onset of action is slower compared to calcitonin. They work by inhibiting bone resorption but take a few days to have a significant effect.
- **Option B: Hydration and diuresis**. Although hydration and diuresis are initial steps in managing hypercalcemia and can help lower calcium levels by increasing urinary excretion, they are not as rapid or potent as calcitonin in severely elevated calcium states.
- **Option D: Chelating agents**. Chelating agents like EDTA can rapidly lower calcium levels but are typically reserved for severe, life-threatening hypercalcemia due to their potential side effects and the availability of safer alternatives like calcitonin.

## Clinical Pearl / High-Yield Fact
A key point to remember is that calcitonin is often used in the emergency management of hypercalcemia due to its rapid onset of action. However, its effect can be short-lived, and it may need to be combined with other treatments for sustained calcium lowering.

## Correct Answer: C. Calcitonin.