Final answer:
To magnetize a ferromagnetic material, atomic magnetic moments within domains must align with an applied external magnetic field, causing domains to grow and align, which can be made permanent with heating, cooling, or mechanical impact.
Step-by-step explanation:
At the atomic level, for a ferromagnetic material to become magnetized, the atomic magnetic moments need to be aligned with the magnetic moments of their domain (Option A). In an unmagnetized ferromagnetic material, the domains are small and have randomly oriented magnetic moments. However, when exposed to an external magnetic field, these domains can grow and align in the direction of the magnetic field. Individual atoms within these domains act like tiny bar magnets, and their poles are aligned accordingly. The magnetization process is reinforced and can become permanent when the material is heated and then cooled, or when it is subjected to mechanical impact while in the presence of a magnetic field.
Answer: A. The atomic magnetic moments need to be aligned with the magnetic moment of their domain.