The correct answer is C. Volume.
Frequency is the number of times something happens in a given period of time. Updating is the process of making something more up-to-date. Volume is the amount or quantity of something.
In the case of a computerized telephone system, the total number of messages handled on a daily basis is an example of volume. This is because it is the total amount of messages that are handled by the system in a given day.
The other options, frequency and updating, are not correct because they do not accurately describe the total number of messages handled by a computerized telephone system on a daily basis.