**Core Concept**
Apoproteins play a crucial role in the regulation of lipoprotein metabolism. They are proteins that bind to lipids to form lipoproteins, which are essential for the transport of lipids in the bloodstream. Among these apoproteins, some have enzymatic activity, while others act as cofactors or receptors.

**Why the Correct Answer is Right**
Apo-C II is a key apoprotein that activates lipoprotein lipase (LPL), an enzyme responsible for the breakdown of triglycerides in lipoproteins, such as VLDL and chylomicrons. Apo-C II binds to LPL and enhances its activity, allowing it to hydrolyze triglycerides into free fatty acids and glycerol. This process is essential for the regulation of lipid metabolism and energy homeostasis in the body.

**Why Each Wrong Option is Incorrect**
**Option A:** Apo-A I is primarily involved in the activation of lecithin-cholesterol acyltransferase (LCAT), an enzyme that converts unesterified cholesterol into cholesteryl esters, rather than lipoprotein lipase.
**Option B:** Apo B48 is a component of chylomicrons and plays a crucial role in the assembly and secretion of triglyceride-rich lipoproteins from the intestine, but it does not activate lipoprotein lipase.
**Option D:** Apo-E is involved in the binding of lipoproteins to the LDL receptor and is also a ligand for the apo-E receptor, but it is not directly involved in the activation of lipoprotein lipase.

**Clinical Pearl / High-Yield Fact**
Apo-C II deficiency is a rare genetic disorder that leads to impaired lipoprotein lipase activity, resulting in hypertriglyceridemia and pancreatitis. This condition highlights the importance of Apo-C II in regulating lipid metabolism and the consequences of its deficiency.

**✓ Correct Answer: C. Apo-C II**