Option (ii) is correct because the defining characteristic of a mechanical wave is that it requires a medium to propagate. The context explicitly states that 'not all waves are mechanical waves, i.e., not all waves need a medium to propagate' and contrasts this with light, which can travel through a vacuum. It further confirms that 'Sound is a longitudinal mechanical wave that needs a medium to travel.' While sound does show reflection, has frequency, and carries energy, these properties can also be exhibited by non-mechanical waves like light. Therefore, the observation that sound needs a medium to propagate best supports its classification as a mechanical wave.