**Core Concept**
The cultivation of hepatitis viruses in vitro is a critical aspect of virology, enabling researchers to study the replication, pathogenesis, and transmission of these viruses. Hepatitis viruses are primarily hepatotropic, targeting the liver cells, but some can be cultured in vitro using specific cell lines.

**Why the Correct Answer is Right**
The correct answer is Hepatitis B virus (HBV). HBV can be cultivated in vitro using a specific cell line, such as HepG2 or Hep3B, which are derived from human liver cancer cells. This is due to the presence of a viral envelope protein that allows the virus to attach and replicate within these cells. The HBV genome can be amplified and analyzed using molecular techniques, providing valuable insights into the virus's life cycle and replication mechanisms.

**Why Each Wrong Option is Incorrect**
**Option A:** Hepatitis A virus (HAV) is not cultivable in vitro because it lacks an envelope protein, making it difficult to attach and replicate within cell lines.
**Option B:** Hepatitis C virus (HCV) is also not cultivable in vitro due to its complex replication machinery and the lack of a suitable cell line that can support its replication.
**Option C:** Hepatitis D virus (HDV) is a satellite virus that requires the presence of HBV for replication and is therefore not cultivable in vitro independently.

**Clinical Pearl / High-Yield Fact**
HBV is a significant public health concern, and its cultivation in vitro has led to the development of effective antiviral therapies, such as nucleoside analogs, which have revolutionized the treatment of chronic hepatitis B.

**Correct Answer: D. Hepatitis B virus**