**Core Concept**
The main LDL receptors are responsible for the clearance of low-density lipoprotein (LDL) cholesterol from the bloodstream. This process is crucial in maintaining normal cholesterol levels and preventing atherosclerosis. The LDL receptor is a transmembrane receptor that recognizes and binds to apolipoprotein B-100 (APO-B-100), a protein component of LDL particles.

**Why the Correct Answer is Right**
APO-B-100 is the primary ligand for the LDL receptor, which is expressed on the surface of hepatocytes in the liver. The binding of APO-B-100 to the LDL receptor triggers a series of intracellular signaling events that ultimately lead to the internalization and degradation of the LDL particle, thereby reducing circulating LDL levels. The LDL receptor is a critical regulator of cholesterol homeostasis, and mutations in the LDL receptor gene can lead to familial hypercholesterolemia, a disorder characterized by elevated LDL levels and premature atherosclerosis.

**Why Each Wrong Option is Incorrect**
**Option A:** APO-A is a protein component of high-density lipoprotein (HDL) particles, not LDL particles. While HDL is involved in reverse cholesterol transport, it is not the primary ligand for the LDL receptor.
**Option C:** There is no such protein as APOc-100. Apolipoproteins are classified into several families, including A, B, C, D, and E, but APOc-100 is not a recognized member of this group.
**Option D:** While APO-E is a ligand for the LDL receptor, it is not the primary ligand. APO-E is also involved in the clearance of other lipoproteins, including chylomicron remnants and VLDL particles.

**Clinical Pearl / High-Yield Fact**
The LDL receptor is a key regulator of cholesterol homeostasis, and mutations in the LDL receptor gene can lead to familial hypercholesterolemia, a disorder characterized by elevated LDL levels and premature atherosclerosis. This highlights the importance of maintaining normal LDL levels to prevent cardiovascular disease.

**✓ Correct Answer: B. APO-B-100**