The correct answer is C. Dummy.
A dummy activity is an artificial activity that is used to show a logical relationship between two activities. It has no duration and no cost. Dummy activities are often used to represent precedence relationships, such as “Activity A must be completed before Activity B can start.”
An event is a point in time at which something happens. Events are often used to represent the start or finish of an activity.
Free float is the amount of time that an activity can be delayed without delaying the project finish date.
A constraint is a restriction that limits the options available to the project team. Constraints can be imposed by the project environment, such as the availability of resources, or by the project management team, such as the project budget.
Here is a diagram that shows an example of a dummy activity:
[Diagram of a project network diagram with a dummy activity]
In this diagram, the dummy activity is shown as a dashed line with an arrow on each end. The dummy activity indicates that Activity B cannot start until Activity A is complete.
I hope this helps! Let me know if you have any other questions.