Atoms in a substance are aligned in a random manner.
A magnetic domain comes into existence when some of the atoms align themselves in a particular direction causing them to behave like a tiny magnet.
A magnetic domain is the alignment of atoms in such a way that they form a tiny magnet within the substance in presence of the external magnetic field.
When a ferromagnetic substance is cooled below Curie temperature, its atoms align themselves in such a way that the aligned atoms in a particular direction behave as tiny magnets.
Thus, correct option is A