**Core Concept**
The diagnosis of acute hepatitis-B infection requires identifying specific markers of the virus in the serum, which indicates a recent infection. HBsAg (Hepatitis B surface antigen) is a key marker that appears early in the course of infection, often before symptoms develop.

**Why the Correct Answer is Right**
The presence of HBsAg in serum indicates that the individual is infected with hepatitis B virus (HBV). HBsAg is a protein on the surface of the virus and is detectable in the blood as early as 1-3 months before the onset of symptoms. Its presence suggests that the individual is in the acute phase of the infection. The body's immune response to HBV also leads to the production of antibodies against the virus, but these appear later in the course of the infection.

**Why Each Wrong Option is Incorrect**
**Option A:** Presence of HbeAg in serum is not specific for acute infection, as it can also be present in individuals with chronic infection who have a high viral load. HBeAg is a marker of viral replication and is often used to assess the infectivity of an individual with chronic HBV infection.
**Option B:** IgM anti-HBc (Immunoglobulin M antibody to hepatitis B core antigen) is a marker of acute HBV infection, but it appears after HBsAg and is not as sensitive or specific for early diagnosis. IgM anti-HBc is often used to confirm acute infection in individuals with a high clinical suspicion.
**Option D:** IgG anti-HBc (Immunoglobulin G antibody to hepatitis B core antigen) is a marker of past or chronic HBV infection and is not specific for acute infection. IgG anti-HBc can persist for many years after recovery from acute infection and is often used to assess immunity to HBV.

**Clinical Pearl / High-Yield Fact**
HBsAg is often used as a screening test for hepatitis B infection, but it is not specific for acute infection. A combination of HBsAg and IgM anti-HBc or HBeAg testing is often used to confirm acute HBV infection.

**✓ Correct Answer: C. Presence HbsAg in serum**