Answer: The correct option is (C). an exterior angle.
Step-by-step explanation: We are to select the name of the new angle that is created by extending one side of a triangle.
Let ABC be a triangle as shown in the attached figure.
∠ABC, ∠ACB and ∠BAC are three interior angles of the triangle ABC.
Let us extend the side BC towards C to point D. The newly created angle is ∠ACD.
∠ACD is called the exterior angle of ΔABC.
Therefore, the newly created angle is an exterior angle.
Thus, option (C) is correct.