**Core Concept**
HLA (Human Leukocyte Antigen) is a group of genes that play a crucial role in the immune system by encoding proteins responsible for the regulation of the immune response. These proteins are involved in the presentation of peptide fragments from inside the cell to the immune system, enabling it to distinguish between self and non-self.

**Why the Correct Answer is Right**
The correct answer is related to the fact that HLA molecules are expressed on the surface of all nucleated cells, where they are responsible for presenting peptide fragments to T-cells. This process is essential for the initiation of an immune response against pathogens. HLA molecules are a type of major histocompatibility complex (MHC) class I molecule, which presents peptides from inside the cell to CD8+ T-cells.

**Why Each Wrong Option is Incorrect**

**Option A:** This option is incorrect because while HLA is present on the surface of immune cells, such as B-cells and T-cells, it is not exclusive to these cells.
**Option B:** This option is incorrect because while HLA is involved in the presentation of peptide fragments to the immune system, it is not directly involved in the process of phagocytosis.
**Option C:** This option is incorrect because while HLA is present on the surface of many cells, it is not exclusively present on the surface of epithelial cells.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that HLA molecules are highly polymorphic, which means that there are many different variants of HLA molecules in the human population. This polymorphism is responsible for the ability of the immune system to recognize and respond to a wide range of pathogens.

**Correct Answer: D. All nucleated cells. HLA is present on the surface of all nucleated cells.**