68. What property of a material is rated in voltage per millimeter (V/mm) of insulation thickness that the insulation can withstand without breaking down?

Dielectric strength is the maximum electric field, typically expressed as voltage per millimeter (V/mm), that an insulating material can withstand before electrical breakdown occurs. Mechanical, tensile, and shear strength describe resistance to mechanical forces, not electrical properties, and are measured in units of stress, not V/mm. Therefore, dielectric strength is the correct answer.