Step 1: Identify the challenge with proteins.
Proteins are macromolecules with very high molar masses (tens of thousands to millions g/mol), so even a concentrated protein solution has an extremely small molar concentration.
Step 2: Compare the sensitivity of each colligative property.
For very dilute solutions, osmotic pressure ($\Pi = MRT$) remains measurable in the order of atmospheres, while boiling point elevation or freezing point depression produce changes smaller than 0.001 K under the same conditions.
Step 3: Consider protein stability.
Methods involving boiling point elevation or freezing point depression require temperature extremes that can denature the protein, destroying its native structure and making the measurement unreliable.
Step 4: Conclusion.
Osmotic pressure is measured at ambient (room) temperature, preserving the protein intact. Its large, easily measurable value at very low molarity makes it the preferred method for determining protein molar mass.
\[ \boxed{\text{Osmotic pressure}} \]