ANS:C - both 'a' & 'b'.

Hard magnetic materials have both high coercive force and high residual magnetic induction. Explanation:

• Coercive force: This refers to the amount of magnetic field strength required to demagnetize a magnetized material. In hard magnetic materials, such as permanent magnets, the coercive force is high, meaning that they require a significant external magnetic field to demagnetize them. This property allows hard magnetic materials to maintain their magnetization over time and makes them suitable for applications where a stable magnetic field is required.
• Residual magnetic induction: Also known as remanence or magnetic flux density, this refers to the magnetic induction remaining in a magnetic material after the removal of an external magnetic field. In hard magnetic materials, the residual magnetic induction is high, meaning that they retain a strong magnetic field even after the external field is removed. This property is essential for permanent magnets to maintain their magnetization and provide a strong magnetic field.