If a mirror is placed on the line AB, then which of the answer figure is the right image of the given figure?

Since, a vertical mirror is placed, thus the top and bottom positions will remain same, while the left and right positions will be swapped.

The black triangle at the top left will appear at top right and thus the second and third figure will be eliminated.

Now, the position of lines in the top right triangle will also change. The lines are parallel to the side of the triangle that does not contain any shaded regions. The mirror image should retain the same relationship. Therefore, option D is the right answer.