**Core Concept**
The underlying principle being tested is the understanding of Hepatitis B virus (HBV) replication and the markers of its active multiplication. HBV is a **hepadnavirus** that infects hepatocytes, and its replication involves the reverse transcription of an RNA intermediate.

**Why the Correct Answer is Right**
The correct answer is related to the indicator of active HBV replication. This involves the **hepatitis B virus DNA** or related markers in the blood, which signify the virus's active multiplication. Specifically, **HBeAg (hepatitis B e-antigen)** is a protein from the HBV that is secreted into the bloodstream when the virus is actively replicating.

**Why Each Wrong Option is Incorrect**
**Option A:** Is incorrect because it does not directly indicate active multiplication of the virus.
**Option B:** Incorrect as it signifies the body's immune response rather than active viral replication.
**Option D:** This option does not directly relate to the active multiplication phase of the HBV lifecycle.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **HBeAg** positivity is often associated with higher viral loads and increased infectivity, making it a critical marker for assessing the risk of transmission and guiding antiviral therapy.

**Correct Answer:** D. HBeAg