Henry's law states that the mole fraction of a gas dissolved in a liquid is directly proportional to the partial pressure of the gas above the solution at constant temperature. Mathematically, p = K_H × x, where K_H is Henry's law constant and x is the mole fraction of the dissolved gas. Given: p = 2.5 × 10⁵ Pa, K_H = 1.67 × 10⁸ Pa. Therefore, x(CO₂) = p / K_H = (2.5 × 10⁵) / (1.67 × 10⁸) = 1.497 × 10⁻³ ≈ 1.5 × 10⁻³. Option 1.5 × 10⁻⁴ results from a factor of 10 arithmetic error, perhaps miscounting powers of 10. Option 3.4 × 10⁻³ comes from incorrectly inverting K_H incorrectly or using the wrong value. Option 6.7 × 10⁻³ results from using K_H = 3.73 × 10⁷ Pa instead of the given value. This directly applies the Henry's law equation from NCERT Chemistry Part I, Chapter 2 (Solutions). Henry's law is particularly important for understanding carbonated beverages and deep-sea diving physiology. Plausibility check: a small mole fraction (10⁻³ range) is expected since CO₂ is sparingly soluble in water under moderate pressures.