Cobalt is the element that possesses a magnetic domain. Magnetic domains are regions within a material where the magnetic moments of atoms or ions are aligned in the same direction. In cobalt, the presence of unpaired electrons in its atomic structure allows for the formation of magnetic domains. These unpaired electrons can align their spins, resulting in a net magnetic moment for the material. As a result, cobalt exhibits ferromagnetic properties, meaning it can be magnetized and retains its magnetization even after the external magnetic field is removed. On the other hand, potassium and lead do not possess unpaired electrons in their atomic structure, and therefore do not exhibit significant magnetic properties.