A guest SSID is a dedicated SSID that is used for guests and other unauthorized users. It is typically a separate SSID from the main corporate SSID and has limited access to the network.
This makes it a good choice for onboarding devices, as it allows the engineer to control the access that the devices have to the network.
The other options are incorrect. A dual SSID is a configuration where two SSIDs are broadcast on the same radio. This is not necessary for onboarding devices. A hidden SSID is an SSID that is not broadcast. This can be useful for security purposes, but it is not necessary for onboarding devices. A broadcast SSID is an SSID that is broadcast. This is the default setting for SSIDs and is not specific to onboarding devices.