Steam distillation is applicable to substances that are immiscible with water, are steam-volatile, and have an appreciable vapour pressure near the boiling point of water. In such a system the total vapour pressure is the sum of the vapour pressures of water and the organic compound acting independently, so the mixture boils when this total equals atmospheric pressure, which occurs below 100 degrees Celsius. This lets aniline distil over with steam at a temperature lower than its own high boiling point of about 184 degrees Celsius, protecting it from decomposition while leaving non-volatile impurities behind. Option B is wrong because steam distillation requires the compound to be immiscible, not miscible, with water. Option C is irrelevant, since density and freezing behaviour do not govern steam volatility. Option D is incorrect because aniline does not react with steam; the separation is purely physical. This is the NCERT rationale for purifying aniline by steam distillation. Plausibility check: because the two immiscible liquids each exert their full vapour pressure, the boiling occurs below the boiling point of either pure component, confirming a gentler distillation.