Check each mixture against the one requirement a separating funnel has, the two components must be immiscible liquids that form separate layers.
Option A, acetone and water: these two liquids are fully miscible, acetone's polar carbonyl group lets it mix completely with water in all proportions, so no layers ever form and a separating funnel is useless here, this pair needs simple or fractional distillation instead.
Option B, oil and water: oil is nonpolar and does not dissolve in polar water, so the two form two distinct layers, water below because it is denser, oil above because it is lighter. A separating funnel is built exactly for this situation, you drain the lower water layer through the stopcock and keep the oil behind, this is the standard example given in every chemistry textbook for separating funnel use.
Option C, different gases in air: air is a homogeneous gaseous mixture, there are no separate liquid layers to speak of at room conditions, this mixture is separated industrially by first liquefying the air and then using fractional distillation, a separating funnel plays no role here.
Option D, none of the above: since option B is a genuine, correct example of separating funnel use, this option cannot be right.
Because oil and water form immiscible layers by density, option B is the correct choice.