A liquid which decomposes at or below its boiling point can be purified by
 

Given data : It is given to identify a liquid which decomposes at or below its boiling point can be purified by

Explanation :

By lowering the pressure above the liquid mixture to below its vapour pressure, vacuum distillation is a distillation technique that causes the most volatile liquid(s) to evaporate. Thus, a liquid which decomposes at or below its boiling point can be purified by distillation under reduced pressure.
 Hence, the correct  option (D). distillation under reduced pressure, is the correct option.