**Core Concept**
The infectivity of Hepatitis B virus (HBV) is commonly diagnosed by detecting the presence of a specific marker that indicates active replication of the virus. This marker is essential for identifying individuals who can transmit the virus to others.

**Why the Correct Answer is Right**
The correct answer is **HBeAg**. HBeAg (Hepatitis B e-antigen) is a protein that is secreted by infected hepatocytes when the virus is actively replicating. The presence of HBeAg in the blood signifies viral replication and increased infectivity. This is because HBeAg acts as a marker of viral replication, and its presence indicates that the virus is actively being produced and released into the bloodstream.

**Why Each Wrong Option is Incorrect**
**Option A:** **HBsAg** (Hepatitis B surface antigen) is a marker of HBV infection, but it does not indicate infectivity. HBsAg can be present in both acute and chronic infections, and its presence does not necessarily signify active viral replication.

**Option B:** **Anti-HBc** (Antibody to Hepatitis B core antigen) is a marker of HBV infection, but it does not indicate infectivity. Anti-HBc can be present in both acute and chronic infections, and its presence does not necessarily signify active viral replication.

**Option C:** **Anti-HBs** (Antibody to Hepatitis B surface antigen) is a marker of immunity to HBV, indicating that an individual has been vaccinated or has recovered from an infection. It does not indicate infectivity.

**Clinical Pearl / High-Yield Fact**
Remember that HBeAg is a marker of viral replication, and its presence indicates that an individual is more likely to transmit the virus to others. This is an essential consideration in clinical practice, particularly when counseling patients about the risk of transmission to others.

**Correct Answer:** C. HBeAg is commonly used to diagnose the infectivity of HBV.