Which of the following substance cannot be separated by magnetic separation?

Out of the four options, iron, nickel, and cobalt are strongly attracted by the magnet. Hence, they are known as ferromagnetic substances. They can be easily separated from the mixture by using a piece of magnet. But aluminium is a nonmagnetic substance and is not attracted to the magnet.

 Hence, fillings of aluminium from a mixture cannot be separated by the process of magnetic separation.