**Core Concept**
MHC class I molecules are a group of proteins that play a crucial role in the immune system by presenting peptide fragments from proteins inside the cell to T-cells. These molecules are expressed on the surface of all nucleated cells in the body, making them a target for cytotoxic T-cells.

**Why the Correct Answer is Right**
MHC class I molecules are encoded by the MHC complex, which is located on chromosome 6 in humans. These molecules are composed of a heavy chain and a light chain, with the heavy chain being non-covalently bound to a beta-2 microglobulin chain. The heavy chain has a groove that can bind to peptide fragments, which are then presented to T-cells. The expression of MHC class I molecules is not limited to immune cells, but is also found on all nucleated cells, including epithelial, endothelial, and muscle cells. This is because MHC class I molecules play a role in the immune surveillance of tumor cells and virus-infected cells.

**Why Each Wrong Option is Incorrect**
**Option A:** All cells is incorrect because MHC class I molecules are not expressed on red blood cells (RBCs), which are nucleated cells that lack a nucleus.
**Option C:** RBCs is incorrect because RBCs do not express MHC class I molecules, despite being nucleated cells.
**Option D:** None is incorrect because MHC class I molecules are indeed expressed on all nucleated cells in the body.

**Clinical Pearl / High-Yield Fact**
It's worth noting that MHC class I molecules are also involved in the presentation of tumor antigens to cytotoxic T-cells, making them a potential target for cancer immunotherapy. Understanding the expression of MHC class I molecules is essential for the development of effective cancer treatments.

**✓ Correct Answer: B. All nucleated cells**