# $\bar{M_N}$ of polymers is always determined by which of the following methods?

$\begin{array}{1 1}(a)\;\text{Light scattering}\\(b)\; \text{Osmotic pressure}\\(c)\;\text{ Sedimentation}\\(d)\;\text{Ultracentrifugation}\end{array}$

For synthetic polymers,$\bar{m_n}$ is always determined by employing methods which depend upon the number of molecules present in the polymer sample.For example,colligative property such as osmotic pressure is used.
On the other hand,weight average molecular mass is measured by using the methods such as light scattering and ultra centrifugation,sedimentation etc.
Hence (b) is the correct answer.