Final answer:
Dummy activities are used in AOA network diagrams to illustrate logical relationships between activities with no duration or resources, represented by a dashed line.
Step-by-step explanation:
The correct answer to the question is: A) Dummy activities.
Dummy activities have no duration and no resources and are used in Activity on Arrow (AOA) network diagrams. These activities are not real actions but are essential for defining the logical relationships between the actual activities in project management. In an AOA network diagram, a dummy activity is represented by a dashed line and is used to show dependencies between tasks when a direct link cannot accurately represent the relationship without causing confusion or an incorrect understanding of task sequences.