Final answer:
The element in question is Cobalt, which is both used in steel production and as a radioactive isotope in cancer treatment.
Step-by-step explanation:
The element being described is a hard, magnetic metal used in steel production and whose radioactive form is used in cancer treatment. The correct answer is C) Cobalt. Cobalt is one of the ferromagnetic materials, which strongly respond to and can be magnetized by magnets. Cobalt-56, a radioactive isotope, plays a role in medical treatments such as cancer therapy due to its emission of gamma rays upon decay.