A(n) __________ receives a weak signal and retransmits it at a higher power.

The term that accurately fills in the blank is "repeater." A repeater is a device designed specifically to receive a weak signal, often due to distance or interference, and amplify it before retransmitting it at a higher power. This process is essential in communication systems, as it helps to extend the range and improve the quality of the signal over long distances or in environments where the signal might degrade.

In various communication contexts, such as telecommunications, networking, and broadcasting, repeaters play a crucial role in maintaining effective and reliable communication by bridging gaps where the signal would otherwise become too weak to be intelligible. By boosting the signal, repeaters ensure that information can be transmitted over longer distances without significant loss of quality.

Other terms like duplex and simplex refer to the method of communication (whether it can occur in both directions simultaneously or just one way) rather than to the amplification and retransmission of signals. An encoder, meanwhile, deals with converting data from one form to another, such as transforming analog audio signals into digital data, and does not serve the function of signal amplification and retransmission.