In a DC generator, what component converts the inherently alternating induced voltage into unidirectional output at the terminals?

Why C is correct

NEETS Mod. 5 §1-1 explains that the commutator — a split ring whose segments reverse the external connections to the loop at the instant the induced voltage would reverse — makes the output unidirectional. Brushes merely maintain sliding contact with the commutator segments; they do not perform the reversal.