Which colligative property is used to …

Molecular mass of polymers are obtained by osmotic pressure and it is used because it it is the most suitable coligative property for the determination of higher molar masses as, the osmotic pressure gives more accurate determination of molar mass than the elevation in boiling point method, it is because, when we see the formulas,
We get for elevation in boiling point , ΔT = K_b . m  (where m = molality) 
So, m =  ΔT / K_b  
Further, moles of solute = molality(m) x mass of solvent in Kg 
Molar mass = mass of unknown molecule/ moles of the unknown molecules
Here, m depends on ΔT , which can vary and so the accurate molar mass is not found.
Whereas, in case of Osmotic pressure(∏) ,  ∏ = MRT ( M = molarity)
So, M = ∏ /RT 
Hence, moles of solute = molality(m) x Volume of solution in L 
Molar mass = mass of unknown molecule/ moles of the unknown molecules
Here, we see that every quantity is fixed and so determination of molar mass will be accurate.