**Core Concept**
Glycoproteins are proteins that have carbohydrate (glycan) chains covalently attached to their polypeptide side-chains. They play crucial roles in various biological processes, including cell-cell interactions, immune responses, and signal transduction.

**Why the Correct Answer is Right**
Glycoproteins can be involved in cell adhesion, signaling, and enzyme functions. They are particularly important in the context of the immune system, where they can serve as receptors or ligands for various immune cells. Examples of glycoproteins include cell adhesion molecules (e.g., selectins, integrins), cytokines (e.g., IL-2, TNF-alpha), and growth factors (e.g., EGF, FGF).

**Why Each Wrong Option is Incorrect**
* **Option A:** This option should be a glycoprotein, but it's not provided. Assuming it's a glycoprotein, it would be correct.
* **Option B:** This option should be a glycoprotein. For example, if it's a cytokine like IL-2, it would be a glycoprotein due to its carbohydrate chains.
* **Option D:** This option should be a glycoprotein. For instance, if it's a growth factor like EGF, it would be a glycoprotein due to its carbohydrate modifications.

**Clinical Pearl / High-Yield Fact**
When considering the structure and function of glycoproteins, remember that their carbohydrate chains can significantly affect their stability, activity, and interactions with other molecules. This is particularly relevant in the context of glycoprotein receptors, where subtle changes in carbohydrate structure can impact ligand binding and downstream signaling.

**Correct Answer: D. Lipopolysaccharide**