The correct answer is 'The user has not logged out and back in since being added to the group'. In Linux, when a user is added to a new group, the group membership is not applied to the user's current sessions. They need to log out and log back in for the system to recognize their new group memberships. 'Full disk space' is incorrect because disk space issues would not affect permissions, 'Filesystem mounted as read-only' is not the likely cause as that would affect all users and not just one, and 'File-specific ACLs' might seem plausible but the question implies group permission is the problem, not specific file permissions.