Assertion(A): Magnetic field lines are confined within the core of a toroid but not within a solenoid.

Reason (R): The magnetic field within solenoid is uniform.

Magnetic field lines can be completely contained within the core of a toroid due to the toroid's closed-loop structure without any ends. This unique geometry ensures that the magnetic field lines form closed loops confined within the toroid's core, creating a concentrated and strong magnetic field.

In contrast, a solenoid, characterized by a coil of wire wound around a cylindrical form, has open ends. As a result, the magnetic field lines inside the solenoid, which run parallel to the length of the coil, cannot form closed curves within the solenoid itself. This can result in some degree of magnetic field "leakage" at the open ends of the solenoid.