**Core Concept**
The question is testing the understanding of the serological markers of hepatitis B virus (HBV) infection, particularly during the acute phase. HBV infection is characterized by the presence of various serological markers, which can be used to diagnose and monitor the infection. The correct answer requires knowledge of the specific marker associated with the acute phase of HBV infection.

**Why the Correct Answer is Right**
During the acute phase of HBV infection, the patient's immune system mounts an immune response against the virus. The primary serological marker of acute HBV infection is the presence of hepatitis B surface antigen (HBsAg) and the development of antibodies against HBsAg (anti-HBs). However, the key marker that differentiates the acute phase from the chronic phase is the presence of IgM antibodies against the hepatitis B core antigen (IgM anti-HBc). This marker is produced in response to the core antigen released during the acute infection and is typically detectable during the first 6 months of infection.

**Why Each Wrong Option is Incorrect**
**Option A:** HBsAg is a marker of HBV infection, but it is not specific to the acute phase. It can be present during both acute and chronic infections.

**Option B:** Anti-HBs is an antibody that appears later in the infection, typically after recovery from the acute phase, and indicates immunity to HBV.

**Option C:** IgG anti-HBc is an antibody that appears later in the infection, typically after the acute phase, and indicates a past or chronic infection.

**Option D:** HBeAg is a marker of viral replication, but it is not specific to the acute phase. It can be present during both acute and chronic infections.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the presence of IgM anti-HBc is a key marker of acute HBV infection, and its detection can help differentiate between acute and chronic infections. This is particularly important in clinical practice, as it can guide treatment decisions and prevent the transmission of HBV to others.

**Correct Answer:** C. IgM anti-HBc.