Which option should be 'OFF' to restrict users from seeing billing rates during time tracking?

To restrict users from seeing billing rates during time tracking, it is necessary to turn off the option that directly relates to the visibility of billing rates. Therefore, the appropriate action would be to disable the setting that states "Show billing rate to users entering time." When this option is off, users will not have access to view the billing rates while they are tracking their time, thus ensuring confidentiality concerning billing information.

The other options do not specifically affect the visibility of billing rates during time tracking. For instance, "Show Service field" pertains to whether users can see the service descriptions, while "Allow time to be billable" focuses on the nature of the time being tracked (billable or non-billable) rather than their ability to see the rates. Lastly, the "First day of work week dropdown" is a setting related to scheduling preferences and has no connection to billing rate visibility. Hence, the correct choice is the one focused on explicitly controlling access to billing rate information.