The correct answer is: C. Circles.
Chance nodes are represented by circles. They represent events that are not under the control of the decision maker. The probability of each event is represented by a number inside the circle.
Disks are used to represent decision nodes. They represent events that are under the control of the
decision maker. The possible choices are represented by branches emanating from the disk.Squares are used to represent terminal nodes. They represent events that have no further consequences. The payoffs for each event are represented by numbers inside the square.
Triangles are not used in decision trees.