the fractions in fractional distillation separate at different... what?

They separate at different boiling points.

Quick scoop

In fractional distillation, each fraction comes out of the column at its own boiling point, because each component in the mixture boils (vaporizes) at a different temperature. This difference in boiling points is what allows the column to separate a complex mixture—like crude oil—into useful fractions such as gasoline, kerosene, and diesel at different heights in the tower.

So the missing word in
“the fractions in fractional distillation separate at different… what?”
is: boiling points.