The answer is (a), the basement or underground garage.
The basement or underground garage provides the best shielding from radiation during an incident because you have the floors above and the ground to provide shielding. If you are told to stay inside, put as many barriers between you and the air. If you don't have an underground area, the center of the middle floor is the next best place to be. This puts floors above and below you. In either case, keep a radio nearby and follow the instructions of your local responders. Learn about Radiation Protection Basics: Time, Distance, Shielding