Benzoic acid in benzene (a non-polar solvent) undergoes dimerisation via hydrogen bonding between the carboxylic acid groups of two molecules: 2C₆H₅COOH ⇌ (C₆H₅COOH)₂. This dimerisation reduces the number of solute particles in solution by half. Since colligative properties depend on the number of particles, a smaller number of particles leads to a smaller observed colligative effect than expected, giving an apparent molar mass that is twice the true value. The van't Hoff factor i = observed particles / expected particles = 0.5 (since 2 molecules become 1 dimer). Option A: Ionisation would give i > 1 (more particles), but benzoic acid ionises in polar solvents like water, not non-polar benzene. Option C: Solvation does not change the number of particles, and i = 2 is incorrect. Option D: Decomposition in benzene is not a known phenomenon for benzoic acid under normal conditions. This is a well-known NCERT example of association leading to abnormally high molar mass. Plausibility check: an elevated apparent molar mass (measured value ≈ 244 g/mol vs actual 122 g/mol) is consistent with dimerisation (factor of 2) and i = 0.5.