## Core Concept
Viruses are obligate parasites that require a host cell to replicate. They consist of genetic material, either DNA or RNA, surrounded by a protein coat known as a capsid. Some viruses also have an outer lipid envelope.

## Why the Correct Answer is Right
The statement in option D regarding viruses is not provided, but generally, correct statements about viruses include their obligate parasitic nature, their structure composed of genetic material and a protein coat, and their classification based on their genetic material and replication strategies.

## Why Each Wrong Option is Incorrect
- **Option A:** Without the specific statement, we can't directly address why it's incorrect. However, if it inaccurately describes a characteristic unique to viruses, such as their requirement for a host cell to replicate or their composition, it would be considered wrong.
- **Option B:** Similarly, without specifics, if this option suggests viruses can replicate outside a host cell or are composed entirely of protein or genetic material without the other, it would be incorrect.
- **Option C:** This option's accuracy depends on its content. If it falsely describes viral replication, structure, or classification, it would be incorrect.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **viruses can be classified based on their type of genetic material (DNA or RNA)**, and this classification influences their replication strategies and the types of diseases they cause.

## Correct Answer: D.