To separate ammonium chloride from common salt, we use:

A mixture of ammonium chloride and common salt can be separated by sublimation. Ammonium chloride is sublime. Therefore, when heated, ammonium chloride converts to vapor leaving common salt. Sublimation is the process by which a solid goes directly to the gaseous state without first going to the liquid state.

Hence the correct option is (D) i.e, sublimation.