A compound which decomposes before its boiling point can be purified by the method of

A compound which decomposes before its boiling point can be purified by the method of distillation under reduce pressure.

When the pressure above the liquid combination to be distilled is dropped to less than its vapor pressure (often less than atmospheric pressure), the most volatile liquid(s) will evaporate. This process is known as vacuum distillation (those with the lowest boiling point).

Hence, option C is correct.