Concept:The distance between two coordinate points is the length of the straight line joining them, calculated using Pythagoras' theorem.Explanation:Let (x1,y1)=(21,21) and (x2,y2)=(−21,−21).Apply the distance formula: D=(x2−x1)2+(y2−y1)2.First, find the change in x: x2−x1=−21−21=−1.Then, find the change in y: y2−y1=−21−21=−1.Substitute these differences into the formula: D=(−1)2+(−1)2.Square each term: (−1)2=1 and (−1)2=1.Add the squares: D=1+1=2.Therefore, the points are 2 units apart.Answer:D=2, so the correct option is D.