ANS:B - curie temperature.

The temperature at which a ferromagnetic material can no longer be magnetized by external forces is termed as the Curie temperature. At temperatures below the Curie temperature, ferromagnetic materials exhibit spontaneous magnetization, meaning they can become permanently magnetized when exposed to an external magnetic field and retain their magnetization after the field is removed. However, as the temperature increases and approaches the Curie temperature, thermal energy disrupts the alignment of magnetic domains within the material, eventually causing it to lose its ferromagnetic properties. Above the Curie temperature, the material becomes paramagnetic, meaning it only exhibits weak magnetic properties in the presence of an external magnetic field but does not retain any magnetization once the field is removed. The Curie temperature is a critical parameter for ferromagnetic materials and varies depending on the composition and structure of the material. It is named after Pierre Curie, a French physicist who conducted pioneering research on magnetism.