**Core Concept**
Viruses are obligate intracellular parasites that lack the machinery to synthesize proteins or replicate DNA/RNA independently. Unlike bacteria, they cannot grow in artificial, non-living environments due to their dependence on host cell machinery.

**Why the Correct Answer is Right**
Viruses require living host cells to replicate because they lack their own metabolic pathways. They must hijack the host’s ribosomes, nucleotides, and energy systems to replicate. No known non-living system provides the necessary cellular environment for viral replication. Thus, viruses can only be cultured in living systems such as embryonated eggs, cell cultures, or animal hosts.

**Why Each Wrong Option is Incorrect**
Option A: Chemically defined media used for bacteria cannot support viral replication because viruses lack metabolic enzymes and cannot generate energy or synthesize macromolecules independently.
Option C: Agar culture is a solid medium used for bacterial growth and does not provide living cells required for viral replication.
Option D: Non-living systems (like cell-free extracts or synthetic media) lack the cellular machinery needed for viral replication, making such culture impossible.

**Clinical Pearl / High-Yield Fact**
Viruses are obligate intracellular parasites — they cannot grow outside a living host. This principle is fundamental in virology and explains why viral infections require host cells for propagation and why viral culture is always performed in living systems.

✓ Correct Answer: B. Living systems only