To solve this problem, we need to understand the process described in the question.$\\ \\$Some solid substances can change directly from a solid state to a vapor state without passing through the liquid state.$\\$This process is known as sublimation.$\\ \\$The technique used for the purification of such substances is based on this principle.$\\$During sublimation, the substance that sublimes is collected as a vapor and then condensed back into a solid,$\\$leaving behind impurities that do not sublime.$\\ \\$Let's analyze the options given:$\\$• Option 1: Distillation - This is used for separating components of a liquid mixture based on differences in boiling points.$\\$• Option 2: Chromatography - This is used for separating components of a mixture based on their movement through a medium.$\\$• Option 3: Crystallization - This is used for purifying solids based on their solubility differences.$\\$• Option 4: Sublimation - This is the correct technique for purifying substances that sublime.$\\ \\$Therefore, the correct option is Option 4: Sublimation.