Recall the definition of Curie temperature

The Curie temperature $T_C$ is the critical temperature above which a ferromagnetic substance loses its spontaneous magnetization due to thermal agitation.

What happens above Curie temperature?

Below $T_C$, ferromagnetic materials have aligned magnetic domains and show strong magnetism.

Above $T_C$, thermal motion becomes strong enough to destroy this long-range alignment, so the material no longer remains ferromagnetic.

It then behaves as a paramagnetic material.

Evaluate the options

• A: a ferromagnetic material becomes paramagnetic
  This is exactly the definition of Curie temperature. ✅

• B: a paramagnetic material becomes diamagnetic
  This is not related to Curie temperature. ❌

• C: a ferromagnetic material becomes diamagnetic
  Above Curie temperature, it becomes paramagnetic, not diamagnetic. ❌

• D: a paramagnetic material becomes ferromagnetic
  This is opposite to the actual behavior. ❌

Final answer

Therefore, the correct option is:

$\boxed{\text{A}}$

Comparison with stored correct answer

Stored correct answer: A
Derived answer: A

Hence, my answer agrees with the stored correct answer.