**Core Concept**
In retinal tamponade, a gas is used to fill the vitreous cavity and support the retina, promoting apposition and reducing retinal detachment. The gas used should have a suitable expansion coefficient, duration of action, and biocompatibility.

**Why the Correct Answer is Right**
The gas used in retinal tamponade is typically sulfur hexafluoride (SF6) or perfluoropropane (C3F8). SF6 has a shorter duration of action (about 2 weeks) and is usually used in cases of retinal detachment with a higher risk of vitreous incarceration. C3F8 has a longer duration of action (about 6 weeks) and is often used in more complex cases or when a longer tamponade is required. Both gases are inert, non-toxic, and biocompatible.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not a gas used in retinal tamponade.
**Option B:** This option is not a gas used in retinal tamponade.
**Option C:** This option is not a gas used in retinal tamponade.

**Clinical Pearl / High-Yield Fact**
When choosing a gas for retinal tamponade, consider the duration of action and potential complications. SF6 may cause a higher risk of cataract formation, whereas C3F8 may be associated with increased intraocular pressure.

**Correct Answer: C. Perfluoropropane (C3F8).**