Final answer:
Simplex communication is the mode where one party must push to talk and release to listen, thus supporting one-directional communication at a time.
Step-by-step explanation:
The mode of communication that requires the EMT to "push to talk" and "release to listen" is known as simplex communication. This type of communication occurs in a one-direction at a time fashion, which means while one person is speaking, they cannot hear the other party until they release the push-to-talk button. In contrast, duplex communication allows for simultaneous two-way communication, multiplex involves multiple signals being combined over a single communication channel, and mediplex is not a recognized term in this context. The mode of communication that requires the EMT to "push to talk" and "release to listen" is duplex communication. In duplex communication, both parties can transmit and receive information simultaneously. This is typically achieved through devices such as two-way radios or walkie-talkies.