In an islsceles triangle two angles are the same and one is different. 40 could be the different angle or the angle there is two of.
The angles in a triangle sum to 180.
If 40 was the double angle, the third would be 180-40*2=100.
If 40 was the different angle the other two would be (180-40)/2=60.
100 is the largest so D is the answer.