• Epidemiology of Relative Risk (RR):
• Definition: Relative risk (risk ratio) measures the strength of association between an exposure and a disease outcome in prospective/cohort studies [Statement A is TRUE].
• Formula: Calculated as the incidence in the exposed ($I_e$) divided by incidence in the unexposed ($I_u$):

Statement B erroneously states it divides unexposed risk by exposed risk [Statement B is FALSE].

• Interpretation: $\text{RR} \gt 1$ signifies increased risk among exposed individuals compared to unexposed [Statement C is TRUE].
• Case-Control Inapplicability: Because case-control studies select subjects based on disease status rather than population denominators, true incidence cannot be computed, making RR uncalculable directly (the Odds Ratio, OR, is used as an estimate) [Statement D is TRUE].

Correct option: (2) (A), (C) and (D) only.

• Diagnostic Test Accuracy Metrics:
• Negative Predictive Value (NPV) is defined as the mathematical probability that an animal testing negative in a diagnostic assay is truly free of the disease:

• Definitions of Other Parameters:
• Diagnostic Sensitivity (DSe): Probability that a diseased animal tests positive ($\frac{TP}{TP+FN}$).
• Diagnostic Specificity (DSp): Probability that a healthy animal tests negative ($\frac{TN}{TN+FP}$).
• Positive Predictive Value (PPV): Probability that an animal testing positive truly has the disease ($\frac{TP}{TP+FP}$).

Correct option: (4) Negative Predictive Value

• 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