The correct answer is A. Modem.
A modem is a device that converts digital signals to analog signals and vice versa. This is necessary for communication over telephone lines, which are designed to carry analog signals. The modem converts the digital signals from your computer into analog signals that can be transmitted over the telephone line. At the other end, the modem converts the analog signals back into digital signals that can be read by your computer.
A digital translator is a device that converts one digital format to another. For example, a digital translator can convert a digital video signal from one format to another.
A router is a device that connects multiple computers to a network. It forwards data packets between computers on the network.
A switch is a device that connects multiple computers to a network. It forwards data packets between computers on the network, but it does not perform any routing functions.
Therefore, the only device that includes an adapter that decodes data sent in radio signals is a modem.