The most strongly ferromagnetic element amongst the following is

Unpaired electrons in ferromagnetic materials like iron, each with a tiny magnetic field of their own, are easily attracted to one another in reaction to an external magnetic field. Even after the magnetic field is turned off, this alignment frequently remains.

Only a small number of substances are ferromagnetic, which is the fundamental mechanism by which certain materials produce permanent magnets and are attracted to magnets. Iron, cobalt, nickel, and the majority of their alloys, as well as several rare earth metal combinations, are the most popular ones.