Virus classification — The primary basis for separating viruses into major groups is:

Introduction / Context:
Modern virology relies on genome type and replication strategy to classify viruses at high levels. This echoes the Baltimore classification and underpins diagnostics and antiviral development.

Given Data / Assumptions:

• Possible criteria include host range, capsid symmetry, and particle size.
• However, genome and replication mode dictate core biology.

Concept / Approach:
At the broadest grouping, genome properties (DNA vs RNA; ss vs ds; positive-sense vs negative-sense; reverse-transcribing) and how genomes are expressed/replicated dominate classification because they determine essential enzymes, replication sites, and susceptibilities to drugs.

Step-by-Step Solution:

Verification / Alternative check:
Reference frameworks consistently group viruses into DNA/RNA and Baltimore classes I–VII, before fine-graining by morphology and host range.

Why Other Options Are Wrong:

• Host nature: many unrelated viruses infect the same host group.
• Capsid symmetry: shared across disparate families.
• Diameter: overlaps widely and is not diagnostic.

Common Pitfalls:
Overemphasizing EM morphology; two morphologically similar viruses can have radically different replication strategies.

Final Answer:
Nucleic acid characteristics (DNA/RNA, sense, strandedness, replication strategy)