**Core Concept**
Glycoproteins are a class of proteins that have carbohydrate (glycan) molecules attached to their peptide chains. These carbohydrate molecules can be covalently linked to the protein backbone through various types of bonds. The attachment of carbohydrates to proteins can affect the protein's structure, stability, and function.

**Why the Correct Answer is Right**
The attachment of carbohydrates to proteins in glycoproteins can serve multiple purposes, including cell-cell recognition, immune response modulation, and enzyme activity regulation. The carbohydrate molecules can also play a role in protein folding and stability, particularly in the context of membrane-bound glycoproteins. The specific type and structure of the carbohydrate molecules attached to a glycoprotein can have significant effects on its function and interactions with other molecules.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because glycoproteins are not a subset of lipoproteins, which are complexes of lipids and proteins. While both types of molecules can be involved in cell-cell recognition and signaling, they have distinct structures and functions.
**Option B:** This option is incorrect because glycoproteins are not simply proteins with a carbohydrate "tag" attached; the carbohydrate molecules are integral to the protein's structure and function.
**Option C:** This option is incorrect because glycoproteins are not a type of nucleic acid; carbohydrates are not a component of DNA or RNA.

**Clinical Pearl / High-Yield Fact**
Glycoproteins play critical roles in various physiological processes, including cell adhesion, immune response, and hormone regulation. Understanding the structure and function of glycoproteins is essential for diagnosing and treating a range of diseases, including cancer, diabetes, and autoimmune disorders.

**Correct Answer: C. Proteins with carbohydrate molecules attached to their peptide chains.**