Step 1: Recall the four colligative properties: relative lowering of vapour pressure, elevation of boiling point, depression of freezing point and osmotic pressure. Each of these depends on how many solute particles are dissolved, not on what the solute is. Osmotic pressure clearly belongs to this family, so the assertion holds.
Step 2: Write osmotic pressure as \(\pi = CRT\). Concentration \(C\) equals moles of solute per litre, i.e. molarity. Keeping temperature constant, doubling the molarity doubles \(\pi\). This proportionality is the reason statement and it is valid.
Step 3: The property is called colligative precisely because \(\pi\) tracks the particle concentration (molarity). So the reason is not just true, it is the underlying cause behind the assertion.
Result: Both are true and the second explains the first, matching alternative (i).