The most important character which suggests that viruses are living is that

Options

A. viruses multiply only in living host
B. their crystals have a definite shape
C. viruses grow in size
D. viruses may be crystallised.

Explanation

**Step-by-Step Solution:**

1. **Understanding Viruses**: Viruses are unique entities that exist at the borderline between living and non-living. They are composed of genetic material (either DNA or RNA) surrounded by a protein coat known as a capsid.

2. **Characteristics of Viruses**: Viruses exhibit certain characteristics that are typically associated with living organisms, such as the ability to reproduce. However, they cannot reproduce independently; they require a host cell to multiply.

3. **Multiplication in Living Cells**: The most significant characteristic that suggests viruses are living is their ability to multiply, but this can only occur inside living cells. When a virus infects a host cell, it uses the cellular machinery of the host to replicate itself.

4. **Inactivity Outside Host Cells**: Outside of a living cell, viruses are inert and cannot multiply. This inactivity further emphasizes their dependence on living organisms for reproduction.

5. **Conclusion**: The ability of viruses to multiply only within living cells is the key characteristic that suggests they are living entities, despite their lack of cellular structure and inactivity outside of host cells.

**Final Answer**: The most important character which suggests that viruses are living is that they multiply only in living cells.

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