The technique used for purification of steam volatile water immiscible substance is

The correct option is Steam distillation.

Explanation:
Steam distillation is a specialized separation and purification technique used for compounds that are heat-sensitive (thermolabile) and meet the following specific criteria:
1. They must be immiscible (or practically insoluble) in water.
2. They must be volatile in steam.
3. They must have a significant vapor pressure at the boiling point of water.

When steam is passed through a mixture containing such a water-immiscible substance, both the water and the organic liquid contribute to the total vapor pressure of the system. According to Dalton's law of partial pressures, the total vapor pressure ($P_{\text{total}}$) of the mixture is the sum of the vapor pressure of water ($P_{\text{w}}$) and that of the organic liquid ($P_{\text{o}}$):
$P_{\text{total}}=P_{\text{w}}+P_{\text{o}}$
Since the mixture boils when $P_{\text{total}}$ equals the atmospheric pressure ($P_{\text{atm}}$), the boiling point of the mixture is always lower than the boiling point of pure water (100 °C) and significantly lower than the boiling point of the organic substance. This allows the organic substance to vaporize and distill over at a much lower temperature, preventing its thermal decomposition.

Analysis of other options:
- Simple distillation: Used for liquids with sufficiently high difference in boiling points (typically greater than 25 °C) and which are stable at their boiling points.
- Fractional distillation: Used to separate miscible liquids whose boiling points are close to each other.
- Distillation under reduced pressure (Vacuum distillation): Used for purifying liquid compounds that decompose at or below their normal boiling points, but does not specifically rely on steam volatility or water-immiscibility.