Which of the following devices converts digital signals to analog ones?

The term 'modem' is short for modulator-demodulator. During transmission, it modulates digital binary data from the computer into analog waveforms for the phone or cable medium. On reception, it demodulates the incoming analog signal back into digital data for the computer - performing the core digital-to-analog and analog-to-digital conversion function.

A switch operates entirely in the digital domain, forwarding Ethernet frames between devices on a LAN based on MAC addresses - it performs no analog conversion.

A hub is a Layer 1 device that broadcasts incoming digital electrical signals to all ports - it does not perform any modulation or analog conversion.

A router forwards digital IP packets between networks based on routing tables - it operates entirely in the digital domain and performs no analog signal conversion.