In a ferromagnetic material, below the curie temperature, a domain is defined as :

Solution

Physics sir · 4 steps

Zoom into one domain: every dipole inside already points the same way, so it is fully magnetised.

1. Inside one domain. Below the Curie temperature, a strong exchange interaction makes neighbouring atomic dipoles line up parallel. Inside a single domain, about 10¹⁷ to 10²¹ atoms all point the same way.
2. Fully magnetised. With all dipoles parallel, the domain has the largest magnetisation the material can have. That maximum value is called the saturation magnetisation.
3. Whole sample. Different domains point in different directions, so an unmagnetised piece can have nearly zero net magnetisation. That zero belongs to the whole sample, not to any one domain.
4. So. A domain is a macroscopic region with saturation magnetisation, which is option (b).

The idea to keep. Each domain is always saturated. An unmagnetised ferromagnet only looks weak because its domains point in different directions. Magnetising it just turns or grows the domains.

Answer from the NTA final answer key. Solution by PrepWiser.