**Core Concept**
Zinc oxide cement, also known as zinc phosphate cement, is a widely used dental restorative material that requires a specific setting time for its optimal application. The setting time of zinc oxide cement is crucial for its proper use in dental procedures. The addition of certain substances can accelerate this process.

**Why the Correct Answer is Right**
The addition of phosphoric acid to zinc oxide cement can accelerate its setting time. Phosphoric acid reacts with zinc oxide to form zinc phosphate, which is the main component of the set cement. This reaction is an acid-base reaction, where the phosphoric acid acts as an acid and the zinc oxide acts as a base. The reaction between phosphoric acid and zinc oxide releases heat, which accelerates the setting process.

**Why Each Wrong Option is Incorrect**
* **Option A:** The addition of water to zinc oxide cement would actually slow down its setting time, as water would dilute the cement and reduce its reactivity.
* **Option B:** The addition of sodium bicarbonate to zinc oxide cement would have no significant effect on its setting time, as it does not participate in the acid-base reaction that sets the cement.
* **Option C:** The addition of citric acid to zinc oxide cement might slightly accelerate its setting time, but it is not as effective as phosphoric acid in accelerating the setting process.

**Clinical Pearl / High-Yield Fact**
Phosphoric acid is a key component in the setting reaction of zinc oxide cement, and its addition can significantly accelerate the setting time. This fact is crucial for dental professionals to remember when using zinc oxide cement in their procedures.

**Correct Answer: C. Citric acid**