What characteristic of a solvent is necessary to avoid oiling out during recrystallization?

The characteristic of a solvent that is necessary to avoid oiling out during recrystallization is having a boiling point lower than the melting point of the solid. This is crucial because if the solvent's boiling point is lower than the solid's melting point, the solid can remain dissolved in the hot solvent during recrystallization. When the solution cools, the solid can crystallize out as the solvent evaporates, allowing for an effective separation of the desired product.

In recrystallization, the goal is to dissolve the solid at high temperatures and then allow it to crystallize upon cooling. If the boiling point of the solvent were higher than the melting point of the solid, it could lead to the solid remaining dissolved even when cooled, or potentially lead to the solid decomposing if it gets too hot, thus preventing effective crystallization and leading to oiling out.

The other options may include factors that are important in specific contexts, but they do not address the primary requirement for preventing oiling out during recrystallization effectively.