Step 1: Definition:
Colligative means "depending on the collection": the property counts particles and ignores their identity.
Step 2: Test:
Osmotic pressure $\pi = CRT$ has no term for the type of solute, only for its concentration.
Step 3: Result:
Vapour pressure, boiling point and freezing point have their own values for a pure liquid, so the colligative effects are the changes in them. Among the options, osmotic pressure is the one that is itself a colligative property, option (C).
Final Answer:
Osmotic pressure is the colligative property.
\[ \boxed{\text{(C) }\text{Osmotic pressure}} \]