The System Administrator is the one who will administer both the AD (users, groups), and will also create a security domain in the container, and set it up to "look" to the specific AD.
The Deployer is the one who will set up the application to use the specific realm, which has been set up by the administrator. Normally, the deployer will not have the rights to define a new realm.
In that respect, I would believe that the correct answers would be b and c, and NOT a and b.
In other words, I would expect c and not a.
The answer a says that:
and the explanation says:The Deployer defines and configures an ADS realm.
I think that defining and configuring a Realm is not the same as assigning it to an application. So, maybe a should be:The Deployer is responsible for assigning the security domain and principal realm to an enterprise bean application.
The Deployer configures the enterprise application to use an ADS realm
and c should be:
The system administrator configures the enterprise application to use an ADS realm.