**Core Concept**
A virus is a small infectious agent that replicates inside the cells of an organism. It is composed of a protein coat and a core of genetic material, either DNA or RNA.

**Why the Correct Answer is Right**
The correct answer is DNA or RNA, as all viruses contain genetic material that encodes for their replication and expression. This genetic material is either single-stranded (ssRNA) or double-stranded (dsDNA, dsRNA), and it is responsible for the synthesis of viral proteins and replication of the viral genome. The genetic material is protected by a protein coat called a capsid, which is composed of multiple copies of a single protein called a capsid protein.

**Why Each Wrong Option is Incorrect**
**Option A:** Glycoproteins are proteins that contain carbohydrate chains and are often present on the surface of viruses, but they are not always present and are not a defining characteristic of viruses.

**Option B:** Lipids are not a universal component of viruses, although some viruses may contain lipid membranes.

**Option C:** Enzymes are not a universal component of viruses, although some viruses may encode for specific enzymes that are necessary for their replication or expression.

**Clinical Pearl / High-Yield Fact**
Viruses can be broadly classified into two main categories: DNA viruses and RNA viruses, based on the type of genetic material they contain. This distinction is important for understanding the replication strategies and disease mechanisms of different viruses.

**Correct Answer:** D.