The correct answer is: C. Sedimentation equilibrium.
Sedimentation equilibrium is a method used to determine the number average molar mass of a macromolecule. It is based on the principle that the rate of sedimentation of a macromolecule is proportional to its mass. The macromolecules are sedimented in a gravitational field, and the concentration of the macromolecules is measured at different heights. The number average molar mass is then calculated from the concentration profile.
Viscosity is a measure of the resistance of a fluid to flow. It is a property of the fluid, not of the solute. The viscosity of a solution of macromolecules is higher than the viscosity of the solvent, but this does not provide information about the molar mass of the macromolecules.
Osmotic pressure is the pressure that must be applied to a solution to prevent the passage of solvent into the solution through a semipermeable membrane. It is a colligative property, which means that it depends only on the number of particles in the solution, not on their identity. The osmotic pressure of a solution of macromolecules is higher than the osmotic pressure of the solvent, but this does not provide information about the molar mass of the macromolecules.
Light scattering is a technique used to study the size and shape of particles. It is based on the principle that when light passes through a solution, some of the light is scattered by the particles in the solution. The amount of light scattered is proportional to the size and shape of the particles. Light scattering can be used to determine the number average molar mass of a macromolecule, but it is not as accurate as sedimentation equilibrium.