Which purification technique is used for high boiling organic liquid compound (decomposes near its boiling point)?

A high-boiling organic liquid that decomposes near its boiling point cannot be purified by simple distillation or fractional distillation, because heating it to its boiling point at normal pressure would cause decomposition. Steam distillation is useful for steam-volatile compounds but is not a general method for all high-boiling liquids.

Reduced pressure (vacuum) distillation lowers the boiling point of the liquid by reducing the external pressure. This allows the compound to boil and be collected at a temperature well below its decomposition temperature. Therefore, the correct purification technique for a high-boiling organic compound that decomposes near its boiling point is reduced pressure distillation.

The answer is option (4).