Those are different devices.
The "Default" will follow whichever is the default audio device in Windows - its an alias for whichever you have currently selected as default, out of the other options below.
The NVIDIA one is for audio over HDMI.
The Realtek one is likely the on-board soundcard with analog output at the mainboard.
You can either use the Default which works for most people, as it automatically just picks the one which is setup in Windows as your audio output, or just pick the right one directly where your audio is supposed to come from (either through HDMI to a AV Receiver, soundbar, or a TV directly), or from the Realtek sound device.