It looks like a config upgrade issue. We were unable to reproduce it, but there are two ways to fix it. Either of these two methods should make all four MAC-address resolution settings reappear in the list: ARP query, NetBios query, WMI query, and Router SNMP MIB query.
If you are okay with losing your current configuration and starting over, simply choose File - Current Config - Reset to Default. This will load the default settings and the current configuration will be lost.
If losing the current configuration is not desirable, close the NetScanner application and manually edit its config. For that, open C:\Users\UserName\AppData\Roaming\SoftPerfect Network Scanner\netscan.xml in a text editor like Notepad and find a section named "macparam". Delete the entire section, including the opening and closing macparam tags. Save the file and start the application again. This part of the config file will be reset to the default settings.