The correct answer to the question is option 4: surface tension. Surface tension is the property of a liquid that allows it to resist an external force that tends to minimize its surface area. In the case of oil and water, both liquids have different molecular structures and polarities. Oil is non-polar and has weak intermolecular forces, while water is polar and has strong intermolecular forces.

The surface tension at the interface between oil and water causes them to separate and not mix together. When oil and water are brought together, the cohesive forces within each liquid are stronger than the adhesive forces between the two liquids. As a result, the surface tension of water forms droplets that repel the oil, creating a distinct boundary between the two substances.

Option 1, bulk modulus, pertains to the compressibility of a material and is not relevant to the mixing of oil and water. Option 2, shear strain, refers to the deformation of a material under shear stress and is also not related to the immiscibility of oil and water. Option 3, volume effect, does not accurately describe the property responsible for the non-mixing of oil and water.

Overall, it is the surface tension in oil and water that prevents them from mixing due to their