**Core Concept**
Soda lime is a crucial component in anesthesia machines and ventilators, responsible for removing carbon dioxide (CO2) from the exhaled breath of patients. This process is essential for maintaining a safe and controlled environment during medical procedures.

**Why the Correct Answer is Right**
The correct answer is **Calcium Hydroxide**. Soda lime is a mixture of calcium hydroxide (Ca(OH)2), water, and a small amount of sodium hydroxide (NaOH). Calcium hydroxide is the primary constituent responsible for the reabsorption of CO2 through a chemical reaction. When CO2 comes into contact with calcium hydroxide, it reacts to form calcium carbonate (CaCO3) and water. This reaction is known as the "CO2 scrubbing" process.

**Why Each Wrong Option is Incorrect**
* **Option A:** Sodium Hydroxide (NaOH) is present in soda lime but is not the primary constituent responsible for CO2 reabsorption. While NaOH can also react with CO2, its role is secondary to that of calcium hydroxide.
* **Option B:** Water (H2O) is a component of soda lime, but it is not directly involved in the CO2 reabsorption process. Water is present to help maintain the correct consistency of soda lime and to facilitate the reaction between calcium hydroxide and CO2.
* **Option C:** No specific substance is mentioned in this option.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that soda lime is a consumable resource and needs to be replaced regularly to maintain its effectiveness in CO2 reabsorption.

**Correct Answer:** C. Calcium Hydroxide