In which mixture is a separating funnel useful?

Correct answer: Oil and water

The direct answer is option A: oil and water. A separating funnel is used for two liquids that do not mix and form separate layers because their densities differ. Oil floats above water, so the lower water layer can be released through the tap and the oil can then be collected separately. Option A is correct because oil and water are immiscible liquids. Option B, salt and water, forms a solution; the salt is dissolved, so a separating funnel cannot separate it. Option C, sugar and water, also forms a solution and needs evaporation or crystallisation for separation. Option D, sand and water, contains an insoluble solid and a liquid; filtration or sedimentation is more suitable, not a separating funnel. The method depends on the physical nature of the mixture, not simply on whether the components look different. Memory cue: two non-mixing liquid layers, such as oil and water, suggest a separating funnel.