Which file, if present, must contain all users that are allowed to use the cron...

The file /etc/cron.allow, if present, must contain all users that are allowed to use the cron scheduling system. This file is used to restrict the access to cron for security reasons. Only users listed in this file can create and edit their own crontab files using the crontab command. If the file does not exist, all users can use cron, unless the file /etc/cron.deny exists, which lists the users that are not allowed to use cron12. References: 1: Cron and Crontab usage and examples. 2: How to use cron in Linux.