1. Identify the problem

We need the best purification technique for a high boiling organic liquid that decomposes near its boiling point.

This means:

• normal boiling is at a high temperature,
• heating to that temperature may cause decomposition,
• so we need a method that allows boiling at a lower temperature.

2. Relevant principle

A liquid boils when its vapour pressure becomes equal to the external pressure.

If we reduce the external pressure, the liquid boils at a lower temperature.

Therefore, compounds that decompose near their boiling points are purified by distillation under reduced pressure (also called vacuum distillation).

3. Evaluate each option

• A: Simple distillation
  Used for liquids with sufficient difference in boiling point from impurities, but it occurs at atmospheric pressure unless specified. Not suitable if the compound decomposes near its boiling point.

• B: Steam distillation
  Used for substances that are steam volatile and immiscible with water, often for essential oils. This is not the general method for all high boiling liquids that decompose.

• C: Fractional distillation
  Used to separate liquids with close boiling points, usually at atmospheric pressure. It does not solve decomposition due to high temperature.

• D: Reduced pressure distillation
  Correct, because lowering pressure lowers the boiling point and prevents decomposition.

4. Final answer

The correct option is:

$\boxed{\text{D: Reduced pressure distillation}}$

5. Comparison with stored answer

Stored correct answer: D
Derived answer: D

They match.