Hint: Split rings are also known as commutators. They are used in DC electric motors to maintain a steady current to the windings which should produce a constant torque since the current direction in a coil is reversed after each half turn.
Complete Step by step solution: Commutators or split rings are metal connections with insulator sheets (of a non-conducting, non-reacting material especially mica) in between for providing a steady direction of current flow. In a power generator, the circuit which moves in the magnetic field produces AC power, since the direction of motion of an arm of the coil changes with respect to the magnetic field after every half cycle. Thus the power output we get is AC power and it can be extracted through slip rings.
However, when needing DC current we would have to change the direction of motion of the coil rotating in the magnetic field after every half cycle. This is not possible as then the motion of a coil in the magnetic field would become oscillatory. This would cause immense mechanical stress every time the motion of the coil is being changed.
Thus instead of changing the motion of the coil, we change the connections after every half-cycle or every \[\pi \,radians\]of angle rotated. This can be done electronically now but earlier split rings were used.
Split rings can be imagined as two-half slip rings with an insulator in between so that the current-carrying conductor which is connected to the coil rotating in the magnetic field, resets its connections after every half cycle of rotation. Thus it gives the output as pure DC, although it is sinusoidal DC current and not a constant voltage. To generate constant DC, many coils of different phases are used in the DC generator.
Therefore the correct option is $(B)$.
Note: Split rings should not be confused with slip rings which are used in AC generators to produce an alternating voltage. Slip rings are used to transfer electrical power from a rotating body to a static body and so were used in commercial power generators.