**Core Concept**
Formaldehyde gas sterilization is a type of chemical sterilization that uses formaldehyde gas to eliminate microorganisms from a specific area or object. This method is effective against a broad spectrum of bacteria, viruses, fungi, and spores.

**Why the Correct Answer is Right**
Formaldehyde gas sterilization works by releasing formaldehyde gas into the environment, which then diffuses into all surfaces and objects, inactivating or killing microorganisms through cross-linking of DNA and proteins. This process is particularly useful for sterilizing rooms, equipment, and surfaces that are difficult to clean or cannot be subjected to high-temperature sterilization methods.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because ethylene oxide sterilization is used for heat-sensitive equipment and materials, not formaldehyde gas sterilization.
**Option B:** Incorrect because gamma radiation sterilization is used for medical devices and equipment that cannot be sterilized by other means, not formaldehyde gas sterilization.
**Option C:** Incorrect because hydrogen peroxide sterilization is used for medical devices and equipment that cannot be sterilized by other means, not formaldehyde gas sterilization.

**Clinical Pearl / High-Yield Fact**
Formaldehyde gas sterilization is often used in hospitals and laboratories to sterilize areas and equipment that are contaminated with highly resistant microorganisms, such as Clostridioides difficile.

**Correct Answer:** C. Medical equipment and surfaces.