Colligative properties depend upon:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → the number of solute particles present in solution.
Colligative properties — relative lowering of vapour pressure, elevation of boiling point, depression of freezing point and osmotic pressure — depend only on the number of solute particles relative to the total number of particles, irrespective of their nature. For example, 1 mol of sucrose and 1 mol of urea in 1 kg of water lower the vapour pressure by nearly the same amount, while 1 mol of NaCl (two ions) has about twice the effect.
Why the other options are incorrect
Option (1) the nature of solvent particles — Incorrect. The solvent fixes constants such as Kb and Kf, but the colligative effect itself is caused by the number of solute particles.
Option (2) the nature of solute particles — Incorrect. The identity of the solute does not matter; equal numbers of different solute particles give the same effect.
Option (4) the physical properties of the solute particles — Incorrect. Size, colour or mass of the solute particles do not enter; only their number counts.
NCERT (2026-27 edition):
Colligative Properties and Determination of Molar Mass – “… All these properties depend on the number of solute particles irrespective of their nature relative to the total number of particles present in the solution. Such properties are called colligative properties (colligative: from Latin: co means together, ligare means to bind).”