Final answer:
The most likely reason for the inability to access the Screen Sharing session is that Screen Sharing is not enabled on Rachel's iMac, though it could also be due to the administrator not being listed as a specific user or the VNC Viewer setting being disabled.
Step-by-step explanation:
The most likely reason that the administrator cannot access the Screen Sharing session on Rachel's Apple iMac is because Screen Sharing is not enabled on her iMac. If Screen Sharing was not activated in the Sharing preferences, no remote user would be able to view or control the iMac. Other potential reasons include the administrator not being included in the Specific Users Only list or the VNC Viewer setting not being enabled, which is necessary if the remote access is attempted from a non-Apple device or from certain remote access software. However, the primary issue generally tends to be that Screen Sharing must be enabled and properly configured on the host computer.