Explanation: Installed Packages is where their Marketing Cloud admin should set up permissions to allow REST authentication. Installed Packages are containers that hold API integrations, components, and permissions in Marketing Cloud. To use the REST API for an external application, the admin needs to create an installed package and configure the API integration settings, such as authentication method, scope, and redirect URI. The admin also needs to assign permissions to the installed package to grant access to specific Marketing Cloud features and data.