Which component in a DC generator makes the inherently alternating induced voltage appear as unidirectional current at the output terminals?

Why B is correct

The commutator is a split ring whose segments reverse the external circuit connections at the instant the induced voltage would reverse, ensuring the brushes always deliver current in one direction. The brushes are the contact point, but the commutator performs the switching action. (NEETS Mod. 5 §1-1)