Final answer:
In UML activity diagrams, looping is best represented using decision points, not synchronization bars, activity ovals, or lines with arrowheads.
Step-by-step explanation:
The question 'Looping in an activity diagram is best represented using what?' is related to UML activity diagrams, a tool used in software engineering to display the sequence of activities. Looping, which allows the representation of repeated activities, is best depicted using decision points. These are shown as diamond shapes within the diagram where the flow of control decides to repeat an activity or move on. The other options mentioned, such as synchronization bars, activity ovals, and lines with arrowheads, serve different purposes in activity diagrams. Synchronization bars are used to join or split flows, activity ovals represent activities themselves, and lines with arrowheads depict the flow from one activity to the next, but not looping.