**Core Concept**
The adaptive mechanism being tested is the ability of the liver to regenerate itself, specifically the process of hyperplasia and hypertrophy of hepatocytes and bile duct cells, allowing the organ to return to its original size after partial resection.

**Why the Correct Answer is Right**
The liver's remarkable ability to regenerate is due to the presence of stem cells and the rapid proliferation of hepatocytes and bile duct cells. This process involves the activation of various signaling pathways, including the Wnt/β-catenin pathway, which promotes cell proliferation and differentiation. The liver's rich blood supply and the presence of growth factors also contribute to its regenerative capacity.

**Why Each Wrong Option is Incorrect**
* **Option A:** Hyperplasia refers to the increase in number of cells, but it doesn't specifically describe the liver's ability to regenerate and return to its original size.
* **Option B:** Hypertrophy refers to the increase in size of cells, but it's not the correct mechanism for the liver's regeneration after partial resection.
* **Option D:** The liver's regeneration is not an example of adaptive immunity, which is the body's defense against pathogens and foreign substances.

**Clinical Pearl / High-Yield Fact**
The liver's ability to regenerate is so remarkable that it can even regenerate from as little as 25% of its original mass, making it an ideal organ for transplantation.

**Correct Answer: C. Compensatory hyperplasia.