The fractional distillation method is used for the purification of liquids when there is a:

Explanation:

The correct option is Small difference (less than 25° C) in their boiling points.

Fractional distillation:

• The method of distillation is used to separate the liquids having sufficient difference in their boiling points. Liquids having different boiling points vaporise at different temperatures, the vapours are cooled and the liquids so formed are collected separately. 
• If the difference in boiling points of two liquids is not much, simple distillation cannot be used to separate them because the vapours of such liquids are formed within the same temperature range and are condensed simultaneously.
• To separate a mixture of two or more miscible liquids for which the difference in boiling points is less than 25ºC, fractional distillation process is used.
  • ​For example, for the separation of different gases from the air, different factions from petroleum products etc.
• Vapours of the liquid with higher boiling point condense before the vapours of the liquid with lower boiling point. By the time the vapours reach to the top of the fractionating column, these are rich in the more volatile component.
• A fractionating column provides many surfaces for heat exchange between the ascending vapours and the descending condensed liquid.
• One of the technological applications of fractional distillation is to separate different fractions of crude oil in the petroleum industry.

Additional Information

• Simple distillation- It is a procedure by which two liquids with different boiling points can be separated. Large difference (greater than 25° C) in the boiling points of liquid

• A vacuum distillation is used when the boiling point of the compound is too high (Tb>150oC) in order to distil the compound without significant decomposition.