Below is the purpose of all the options.
Keep AP's setting: By default whatever IP mode is set on AP (by default all APs will be set to dual mode - dynamic).
Manual/Static: You can change the IP and statically define it.
DHCP: Once selected, if AP was previously set with a static IP, it will be forced to get IP from DHCP.
So if your network has DHCP and AP is already functional, you can keep the setting or you can choose DHCP.