**Core Concept**
Soda lime is a critical component in anesthesia machines, used to remove carbon dioxide (CO2) from exhaled breath in a closed circuit. The reabsorption of CO2 is a vital process to maintain a safe breathing environment for patients undergoing anesthesia.

**Why the Correct Answer is Right**
The most important constituent in soda lime for reabsorption of CO2 is calcium hydroxide (Ca(OH)2). Calcium hydroxide reacts with CO2 to form calcium carbonate (CaCO3) and water, effectively removing CO2 from the breathing circuit. This reaction is catalyzed by the presence of potassium hydroxide (KOH) in soda lime, which helps to regenerate the calcium hydroxide by breaking down the calcium carbonate. The reaction is as follows:

Ca(OH)2 + CO2 → CaCO3 + H2O

**Why Each Wrong Option is Incorrect**
* **Option A:** Sodium hydroxide (NaOH) is not the primary constituent responsible for CO2 reabsorption in soda lime. While it may be present in small amounts, its role is secondary to calcium hydroxide.
* **Option B:** Potassium hydroxide (KOH) is essential for catalyzing the reaction between calcium hydroxide and CO2, but it is not the most important constituent in soda lime.
* **Option D:** There is no commonly recognized constituent in soda lime by this name.

**Clinical Pearl / High-Yield Fact**
Soda lime should be replaced regularly to maintain its effectiveness in removing CO2 from the breathing circuit. Failure to do so can lead to CO2 accumulation, resulting in respiratory acidosis and other serious complications.

**Correct Answer:** C. Calcium hydroxide.