Final answer:
A radio transceiver is the portable communication device used by firefighters for critical communication during operations. Other options such as a radiometer, radiosonde, and radiocarbon are not related to communication but rather to radiation measurement, atmospheric data collection, and dating materials respectively.
Step-by-step explanation:
The portable communication device used by firefighters is known as a radio transceiver. This device allows the firefighting team to communicate important information like distress signals, reporting progress, and alerting to changes in fire conditions. The other options presented, such as radiometer, radiosonde, and radiocarbon, are not communication devices. A radiometer measures the intensity of radiation, a radiosonde is a battery-powered telemetry instrument carried into the atmosphere usually by a weather balloon that measures various atmospheric parameters and transmits them to a ground receiver, and radiocarbon is a technique used for dating materials. In contrast, a radio transceiver combines both transmitter and receiver in a single unit, allowing for two-way communication, which is essential for the safety and coordination among firefighters.