The suggested answer is A, D.
To invite a new user to administer specific sites in read-only mode, the roles available are Helpdesk and Observer. The Helpdesk role provides read-only access to allowed sites, assisting in resolving user issues without the ability to modify configurations. The Observer role also offers read-only access, allowing users to view site details without making changes. Other roles such as Network Admin and Installer typically have elevated privileges that include the ability to make changes, which does not align with the read-only requirement.