The assertion A is correct because an electric fan continues to rotate for some time after the current is switched off. This is due to inertia of motion, which is the tendency of an object to resist changes in its state of motion. Even after the current is switched off, the fan blades continue to rotate for a while.
The reason R is also correct because the fan continues to rotate due to inertia of motion. Inertia is the property of mass due to which an object continues to move until an external force acts upon it to stop the motion.
Thus, both Assertion A and Reason R are correct, and Reason R correctly explains Assertion A.