**Core Concept**
The diagnosis of acute hepatitis B virus (HBV) infection relies on detecting specific antibodies that indicate a recent infection. HBV infection causes the production of hepatitis B surface antigen (Hbs Ag) and hepatitis B core antigen (Hbc Ag), leading to the formation of antibodies against these antigens.

**Why the Correct Answer is Right**
The correct answer involves the detection of IgM antibodies against Hbc Ag, which are indicative of an acute HBV infection. This is because IgM antibodies are the first line of defense and typically appear early in the course of an infection. The presence of IgM anti-Hbc Ag indicates that the individual has recently been infected with HBV. The core antigen is not directly detectable in the blood, but the presence of IgM antibodies against it is a reliable marker of acute infection.

**Why Each Wrong Option is Incorrect**
**Option A:** IgM anti-Hbs Ag is not a reliable marker for acute HBV infection. While IgM antibodies against the surface antigen can be detected in some cases, they are not as specific or sensitive as IgM anti-Hbc Ag for diagnosing acute HBV infection.

**Option C:** IgG anti-Hbs Ag is a marker of past infection or immunity to HBV, rather than acute infection. IgG antibodies against the surface antigen typically appear later in the course of infection and persist for life, indicating either past infection or vaccination.

**Option D:** IgG anti-Hbc Ag is a marker of past infection, not acute infection. IgG antibodies against the core antigen typically appear later in the course of infection and persist for life, indicating either past infection or exposure to HBV.

**Clinical Pearl / High-Yield Fact**
When diagnosing acute HBV infection, it's essential to remember that IgM anti-Hbc Ag is the most reliable marker of acute infection. This is because IgM anti-Hbc Ag is specific for acute infection and can be detected in the blood as early as 1-2 weeks after exposure to HBV.

**✓ Correct Answer: B. IgM antibody of Hbc Ag**