Option D is correct because the textbook explicitly states that 'a canyon is characterised by steep step-like side slopes.' The context clearly distinguishes between different valley types: a gorge is described as 'a deep valley with very steep to straight sides,' while a canyon specifically has 'steep step-like side slopes.' Additionally, the text notes that a canyon is wider at its top than at its bottom and is a variant of gorge, commonly forming in horizontal bedded sedimentary rocks.