The correct option for the purification of steam volatile water-immiscible substances is Option D, Steam distillation.

Steam distillation is a separation technique used to purify or isolate temperature-sensitive materials, such as natural aromatic compounds, that are volatile in steam at a temperature that is lower than their decomposition temperatures. It exploits the fact that immiscible liquids can be distilled at a lower temperature than the boiling points of the individual components.

Here is how steam distillation works:

• Water and the other immiscible substance are placed in the distillation apparatus.
• Steam is generated either within the same flask or introduced from an external source.
• The hot steam helps vaporize the other substance at a temperature below its normal boiling point.
• The vapors of water and the volatile substance both pass into the condenser, where they are cooled.
• Upon cooling, the vapors convert back into liquid, which is then collected in a separate container.
• Since the two substances are immiscible, they separate into two distinct layers, typically, with the organic compound floating on top of the water or sinking below it, depending on their densities.
• The layers can then be separated using a separating funnel or other means.

Steam distillation is different from fractional distillation (Option A), which is used for the separation of a mixture into its component parts, or fractions, of compounds that are miscible and have different boiling points. Fractional distillation often involves the use of a fractionating column, which brings more efficiency in separating substances with close boiling points.

Simple distillation (Option B) is suitable for separating a liquid from impurities or for separating liquids with significantly different boiling points (usually the difference in boiling points should be at least $25^\circ C$).

On the other hand, fractional distillation under reduced pressure (Option C) is used when a liquid has a very high boiling point or might decompose at high temperatures. By reducing the pressure, the boiling point of the liquid is lowered and it can be distilled at a temperature much lower than its normal boiling point, thus avoiding decomposition.

Therefore, for volatile substances that are not miscible with water, steam distillation remains the ideal choice.