Which of the following substances is not miscible with water?

Miscibility describes whether two liquids merge without separating into distinct layers when mixed. For a true solution, this is governed by the principle "like dissolves like": water is a polar solvent that forms extensive hydrogen bonds with other molecules, so a liquid dissolves completely into water only when its own molecules can form comparable hydrogen bonds or polar interactions with water; a liquid whose molecules are nonpolar cannot match this attraction and instead separates out as its own layer. A stable dispersion (an emulsion) is a separate case: it can stay mixed even when the dispersed substance itself is not water-soluble, as long as its droplets are kept finely suspended within a continuous aqueous phase.

Checking each option against this principle:

• Milk is an emulsion of fat droplets kept finely suspended within a mostly aqueous continuous phase, so when mixed with more water it stays dispersed as a uniform-looking mixture rather than separating into a layer.

• Honey is a concentrated solution of sugars — small, hydroxyl-rich, polar molecules — in water, so it readily dissolves into water when mixed.

• Alcohol (ethanol) carries a hydroxyl (-OH) group that hydrogen-bonds directly with water molecules, so it mixes with water in all proportions.

• Oil is a triglyceride: long hydrocarbon (fatty-acid) chains joined to a glycerol backbone by ester linkages. Its small polar ester groups are vastly outweighed by the long nonpolar hydrocarbon chains, so oil cannot engage water's hydrogen-bonding network and instead stays as a separate layer rather than blending in.

This matches the everyday observation that oil floats as a distinct layer on top of water, while alcohol, honey, and milk each disappear into the water on stirring — confirming that oil is the substance that is not miscible with water.