Which of the following is a dummy activity ? 

Explanation :

Dummy activity:

• While establishing the relationships and drawing the network diagram sometimes it so happens that there does not exist any real activity between two events but an event cannot be said to have occurred unless and until some other event has occurred.
• Such cases are shown on the network diagram as dummy activities.
• The purpose of dummy activity is to maintain the logic of a network diagram.
•  Since no real activity takes place between two such events, the dummy activity does not consume any resources.
•  It also does not consume time duration.
• Dummy activities are used only in case of absolute necessity.

Here, the dummy activity is between 2 - 3 which is given by dotted lines.

• Excavate the foundations: This will be an internal activity (a part of the project which requires some duration and cost as well to reach the tail event.
• Waiting for the arrival of concrete materials: It won't require any cost or any time hence it can be called a dummy activity.
• Lay the foundation concrete: This is the second stage after the excavation which will require money and time hence not a dummy activity
• Cure the foundation concrete: This is the third stage after the lay of concrete which requires minimum of 7 days of curing and manpower & resource cost.