Explanation: The ATR (Automatic Transmit-Receive) box, also known as a T/R switch, is a critical component in a transceiver that shares a single antenna for both transmitting and receiving. Its primary function is to switch the antenna between the transmitter and the receiver, ensuring proper signal routing and protecting sensitive components. **Option B is correct:** During receive mode, the ATR box connects the antenna to the receiver and simultaneously isolates the transmitter. This prevents the weak incoming signal from the antenna (the "received signal") from being diverted into the transmitter's output circuitry, where it would be shunted or attenuated, thereby preventing it from reaching the receiver's input. **Option A is incorrect:** While receiver protection is a general concern, the ATR box's specific role isn't to filter or protect against strong *external* signals like RADAR. Its primary protection function relates to the transceiver's *own* transmit power. **Option C is incorrect:** The ATR box physically switches the antenna connection. It doesn't "turn off" the receiver circuit itself. While the receiver is often muted or desensitized during transmission to protect it from the local transmit signal, this is a separate control function, not the direct action of the ATR switch.