A centralized switching system is a telephone exchange in which the switching fabric (the network that physically connects one subscriber line to another) and the intelligence that governs it are concentrated in one location. The brain of such an exchange is the Control Unit — in modern exchanges typically a stored-program-controlled processor. Its job is to interpret subscriber signalling and orchestrate every stage of a telephone call.

Specifically, the Control Unit is responsible for:

• Receiving and processing the dialed digits from the calling subscriber, then performing digit analysis to determine the destination.
• Making routing decisions and selecting a free path through the switching matrix.
• Call setup, supervision, and teardown — establishing the connection, monitoring it for the duration of the conversation, and releasing the resources when either party hangs up.
• Coordinating auxiliary subsystems (such as commanding the ringing generator to alert the called party).

Therefore the description processing dialed digits and managing call setup correctly captures the Control Unit's role. The remaining options describe separate, dedicated subsystems: generating the ringing voltage is done by a specialized ringing generator (the Control Unit merely commands it), the caller's voice is produced by the human speaker and carried through the transmission/speech path (not generated by any control logic), and physically moving copper wires is not how switching works at all — connections are made electronically or by switch contacts, not by relocating conductors.