The luster of a metal is due to A. its high density B. its high polishing C. its chemical inertness D. presence of free electrons

The correct answer is D. presence of free electrons.

The luster of a metal is due to the presence of free electrons. These electrons are able to move freely within the metal, and they reflect light in a way that produces a shiny appearance.

Option A is incorrect because the density of a metal does not affect its luster.

Option B is incorrect because the polishing of a metal does not affect its luster.

Option C is incorrect because the chemical inertness of a metal does not affect its luster.