**Core Concept**
HLA-I (Human Leukocyte Antigen class I) molecules are a group of proteins that play a crucial role in the immune system by presenting endogenously synthesized peptides to CD8+ T cells. They are expressed on the surface of almost all nucleated cells, including epithelial cells, endothelial cells, and fibroblasts.

**Why the Correct Answer is Right**
HLA-I molecules are essential for the recognition and elimination of virus-infected cells and tumor cells by CD8+ T cells. They are composed of a heavy chain (α) and a light chain (β2-microglobulin) and present peptides derived from proteins that are synthesized within the cell, including viral proteins and tumor antigens. The presentation of these peptides by HLA-I molecules to CD8+ T cells activates a cytotoxic T cell response, which is critical for the elimination of infected or malignant cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because HLA-II molecules are primarily expressed on the surface of antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells, and are involved in the presentation of exogenously derived peptides to CD4+ T cells.
**Option B:** This option is incorrect because while some cells may express both HLA-I and HLA-II molecules, HLA-II molecules are not present on the surface of all nucleated cells.
**Option C:** This option is incorrect because platelets do not express HLA-I or HLA-II molecules.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that HLA-I molecules are expressed on the surface of almost all nucleated cells, making them a critical component of the immune system's ability to recognize and eliminate infected or malignant cells.

**Correct Answer: C. Platelets.**