Step 1: Approach
Ask which listed quantity is, by name, one of the four colligative properties.
Step 2: The list of four
Relative lowering of vapour pressure, elevation in boiling point, depression in freezing point, osmotic pressure.
Step 3: Matching
Option (B) appears directly as the fourth item. The other three options name the vapour pressure, boiling point and freezing point of the solution as bare values, whereas the colligative properties are the changes in them.
Step 4: Formula check
Osmotic pressure $\pi = CRT$ contains only particle concentration, with no term for the identity of the solute. That is the mark of a colligative property.
Final Answer:
Osmotic pressure depends only on the number of dissolved particles, option (B).
\[ \boxed{\text{Osmotic pressure}} \]