Assertion (A) states that individual atoms in a ferromagnetic material possess a magnetic dipole moment and interact to form domains. This is a correct description of the domain theory of ferromagnetism, where exchange interactions lead to spontaneous alignment of moments within small regions called domains.
Reason (R) states that at high temperatures (above the Curie temperature TC), the domain structure disintegrates due to thermal agitation, and the material becomes paramagnetic. This is also a correct statement regarding the temperature dependence of ferromagnetic materials.
However, Reason (R) describes the effect of temperature on the domain structure, whereas Assertion (A) describes the fundamental nature and formation of domains in the ferromagnetic state. The disintegration of domains at high temperature is not the reason why they form spontaneously at lower temperatures. Therefore, both statements are true, but (R) is not the correct explanation of (A).