Step 1: Two solutions are isotonic when they exert equal osmotic pressure. Osmotic pressure is a colligative property that is governed only by how many solute particles are present per unit volume, so equal osmotic pressure implies equal particle concentration.
Step 2: Boiling point elevation is another colligative property. All colligative properties (relative lowering of vapour pressure, boiling point elevation, freezing point depression, osmotic pressure) rise or fall together because they all track the same particle concentration.
Step 3: Therefore, if the particle concentration is fixed (as it is for isotonic solutions in the same solvent), the boiling point elevation must also be fixed and identical for both. The assertion holds.
Step 4: The reason gives the underlying cause: since boiling point (its elevation) is set by solute concentration, equal concentration forces equal elevation. Reason is true and is the genuine explanation.
Answer: Both true, reason explains assertion, so the correct choice is the first alternative.