The suggested answer is A, C, E.
There are three main ways to add plugins to the base platform. First, plugins can be activated by an admin if they are not already active by default. Second, plugins can be requested from ServiceNow if they are not included in the base platform. Third, plugins can also be activated by specific roles such as an IT Portfolio Manager (it_portfolio_manager) or an IT PPS Admin (it_pps_admin), with 'it_pps_admin' being one of the appropriate roles. Plugins that are activated by default in the base ServiceNow system would not need to be 'added' in the traditional sense, so that option does not align with how plugins are 'added' in a new or inactive state.