**Core Concept**
Plasma sterilization is a low-temperature method using a gas plasma to inactivate microorganisms. It involves the use of a non-thermal plasma to break down organic molecules, thereby inactivating bacterial spores and other microbial forms. This process is widely used in medical and pharmaceutical industries for sterilizing equipment and supplies.

**Why the Correct Answer is Right**
The correct answer is hydrogen peroxide (H2O2). Hydrogen peroxide is used as a control in plasma sterilization due to its ability to act as a marker of the effectiveness of the sterilization process. When a plasma sterilization process is validated, hydrogen peroxide is used to demonstrate the efficacy of the method. The hydrogen peroxide is introduced into the sterilization chamber and its breakdown is monitored to ensure that the plasma is functioning correctly. If the hydrogen peroxide breaks down, it indicates that the plasma is active and the sterilization process is effective.

**Why Each Wrong Option is Incorrect**
**Option A:** Formaldehyde is not used as a control in plasma sterilization, although it is a common sterilizing agent. It has a distinct mechanism of action and is not used in conjunction with plasma sterilization.
**Option B:** Ethylene oxide is a widely used sterilizing agent, but it is not used as a control in plasma sterilization. It requires a different type of validation and monitoring.
**Option C:** Glutaraldehyde is a potent disinfectant and sterilizing agent, but it is not used as a control in plasma sterilization. It is not compatible with the plasma process and would not be a suitable control.

**Clinical Pearl / High-Yield Fact**
When validating a plasma sterilization process, hydrogen peroxide is used as a control due to its ability to break down in the presence of a functioning plasma. This makes it an ideal marker of the effectiveness of the sterilization process.

**Correct Answer:** C. Hydrogen peroxide (H2O2)