The correct answer is B) Dielectric strength. Dielectric strength refers to the voltage per millimeter of insulation thickness that the insulation can withstand without breaking down or failing. This is a critical property for electrical components and systems, as it determines the insulation's ability to prevent electrical current from flowing through it unintentionally. The other options are incorrect because they do not specifically describe the voltage-to-thickness ratio of an insulator's breakdown point. Tensile strength (A) refers to the material's ability to withstand pulling forces, mechanical strength (C) is a general term for a material's resistance to deformation, and shear strength (D) is the material's resistance to parallel forces that can cause layers to slide past each other.