Final answer:
Duration drivers are used when the time required for activities varies per transaction, accurately linking resource consumption with the time spent on each activity.
Step-by-step explanation:
When the time required to perform an activity varies from transaction to transaction, the most appropriate activity driver to use would be c. Duration drivers. Duration drivers are activity drivers that do a better job of linking the consumption of resources with time-based measurements. For example, in a service where the time spent on customer support varies depending on the complexity of the customer's issue, a duration driver would provide a more accurate method of assigning costs based on the actual time spent on each transaction.