The correct answer is D. Relative density has no unit. It is a dimensionless quantity. It is defined as the ratio of the density of a substance to the density of a reference substance. The reference substance is usually water, which has a density of 1 g/cm3 or 1000 kg/m3. Relative density is often used to compare the densities of different substances. For example, the relative density of gold is 19.3, which means that gold is 19.3 times denser than water.
Option A is incorrect because kilograms per meter (kg/m) is the unit of mass per unit length. Option B is incorrect because kilograms per square meter (kg/m2) is the unit of mass per unit area. Option C is incorrect because kilograms per cubic meter (kg/m3) is the unit of mass per unit volume.