• Neutralizing Residual Chlorine: Sodium Thiosulphate ($\text{Na}_2\text{S}_2\text{O}_3$, typically added at $0.1\text{ ml}$ of a $10\%$ solution per $100\text{ ml}$ sample bottle) is the standard reducing chemical used to instantly neutralize residual free and combined chlorine in treated drinking water.
• Microbiological Rationale: Chlorine neutralization is critical when collecting water samples for bacteriological examination (e.g., coliform counts) to prevent continued bactericidal action during transit to the laboratory, which would cause false-negative culture results.

Correct option: (4) Sodium Thiosulphate