**Core Concept**
The liver is a highly vascular organ prone to various types of tumors, including benign and malignant lesions. The most common tumor of the liver has a distinct histological characteristic and is often associated with a specific demographic.

**Why the Correct Answer is Right**
The correct answer is Hepatocellular Carcinoma (HCC), which accounts for approximately 85-90% of primary liver cancers. HCC arises from the hepatocytes, the main cell type of the liver, and is often linked to chronic liver disease, hepatitis B and C infection, and cirrhosis. The pathogenesis of HCC involves the activation of oncogenes and the inactivation of tumor suppressor genes, leading to uncontrolled cell proliferation and tumor formation.

**Why Each Wrong Option is Incorrect**
* **Option A:** Focal Nodular Hyperplasia (FNH) is a benign tumor of the liver, composed of normal liver cells and fibrous tissue. It is not a malignant tumor and is not the most common type of liver tumor.
* **Option B:** Hemangioma is a benign vascular tumor of the liver, composed of blood vessels and is not a malignant tumor. It is the most common benign liver tumor but not the most common liver tumor overall.
* **Option C:** Adenoma is a rare benign tumor of the liver, composed of glandular cells and is not the most common type of liver tumor.

**Clinical Pearl / High-Yield Fact**
The risk of hepatocellular carcinoma is significantly increased in patients with chronic liver disease, particularly those with hepatitis B and C infection, and cirrhosis.

**Correct Answer:** D. Hepatocellular Carcinoma