**Core Concept**
The Australian antigen, also known as hepatitis B surface antigen (HBsAg), is a protein found on the surface of the hepatitis B virus (HBV). It is a key marker for the diagnosis of acute and chronic hepatitis B infection.

**Why the Correct Answer is Right**
The Australian antigen is a protein that forms the outer envelope of the hepatitis B virus. It is composed of three different proteins: large (L), middle (M), and small (S). The S protein is the most abundant and is the one that is used as a marker for the virus. The presence of HBsAg in the blood indicates that the person is infected with HBV. It is the first detectable antigen in the serum, appearing before the onset of symptoms and persisting in the serum for several months.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the protein or the virus in question. The Australian antigen is a specific marker for HBV, not a general term for any protein.
**Option B:** This option is incorrect because it is a different protein associated with HBV, but not the one used as a marker for the virus.

**Clinical Pearl / High-Yield Fact**
HBsAg can persist in the serum for several months after acute infection, and its presence is a marker for chronic infection if it remains present for more than six months.

**Correct Answer:** C. Hepatitis B surface antigen (HBsAg).