Benzenediazonium chloride is a colourless solid that decomposes in

Dry state

The correct answer is Option (1) → Dry state

• Benzenediazonium chloride (C₆H₅N₂⁺ Cl⁻) is highly unstable in the dry solid state. When isolated as a dry solid, it decomposes explosively even at room temperature or slightly above, often with evolution of nitrogen gas:

C₆H₅N₂⁺ Cl⁻ → C₆H₅Cl + N₂ ↑ (or other products)

That is why it is never isolated in dry form in laboratories.

• In cold water (below 5–10°C), it is relatively stable when kept in solution (especially with a little acid). This cold aqueous solution is used for most diazonium reactions (Sandmeyer, coupling, etc.).
• In warm water, it decomposes slowly to give phenol:

C₆H₅N₂⁺ + H₂O → C₆H₅OH + N₂ + H⁺

But this decomposition is controlled and non-explosive.

Summary:

• Explosive decomposition → Dry state
• Stable → Cold dilute aqueous solution
• Controlled decomposition → Warm/hot water

Thus, the correct answer is: Dry state