The Graph Options are not related to any options to the actual graphs, but describe the type of view on the graphs which the user has access to. There are three different options available:

These views relate to the small tabs on the upper right of the Cacti screen when you are using the Graphs tab:

The Authentication Realm defines the type of access authentication used when a user logs in to Cacti. There are three different options available:

The Local realm uses the built-in authentication from Cacti. LDAP authentication can authenticate users against any LDAP server including Active Directory servers. Web Basic authentication is handled by the web server itself (for example, using .htaccess files)