Explanation:

- Red wire: This is just a type of colored wire. It’s not a device and doesn’t convert signals.

- Blue cord: This is simply a cable, often used for certain network connections, but it doesn't convert digital signals to analog.

- Tower: This refers to the physical case or chassis of a desktop PC. It houses components but doesn’t handle signal conversion.

- Modem: Correct Answer

- A modem (short for modulator-demodulator) converts your computer's digital data to analog signals that can travel over telephone lines and vice versa.

- This is key for dial-up internet and some broadband services.