Which of the following is not one of the three primary functions that on-line direct-access systems can serve?

The correct answer is D. programming.

On-line direct-access systems are computer systems that allow users to access and update data directly, without the need for a programmer to write a program. The three primary functions of on-line direct-access systems are:

• Inquiry: Users can query the system to retrieve data.
• Update: Users can update data in the system.
• Backup: Users can back up data from the system.

Programming is not one of the three primary functions of on-line direct-access systems because it is not necessary for users to be able to write programs in order to use the system.