The osmotic pressure method is particularly useful for determining th…

The correct answer is Option (2) → are generally not stable at higher temperatures

In the osmotic pressure method the pressure is measured at about room temperature. Biomolecules such as proteins are generally not stable at higher temperatures, so a method that needs no heating suits them.

Why the other options are incorrect

Option (1) associate in solution, which raises the osmotic pressure — Incorrect. Association reduces the number of particles in solution (i is less than 1), so it would lower the osmotic pressure, and it is not the reason the method is used.

Option (3) give a large depression in freezing point even in very dilute solutions — Incorrect. A dilute solution of a solute of large molar mass gives only a very small depression in freezing point. It is the osmotic pressure whose magnitude is large even for very dilute solutions.

Option (4) are volatile and pass into the vapour along with the solvent — Incorrect. Biomolecules are non-volatile solutes; they do not pass into the vapour.

NCERT (2026-27 edition):
1.6.4 Osmosis and Osmotic Pressure – “…The osmotic pressure method has the advantage over other methods as pressure measurement is around the room temperature and the molarity of the solution is used instead of molality. As compared to other colligative properties, its magnitude is large even for very dilute solutions. The technique of osmotic pressure for determination of molar mass of solutes is particularly useful for biomolecules as they are generally not stable at higher temperatures and polymers have poor solubility.”

1.7 Abnormal Molar Masses – “…In case of association, value of i is less than unity while for dissociation it is greater than unity.”