Select the image where the dotted line is a line of symmetry.

If we keep a mirror vertically over the image of option D through the middle i.e through the dotted lines, we can get a perfect mirror image which looks exactly the same as the image of option D.
The line of symmetry of the image is vertically through the middle.
Therefore, the correct option is option D

For other options, if we keep a mirror along the dotted lines, we won't be getting a perfectly similar mirror image as one side of the dotted line will be different from the other side of the dotted line. We can say that the line of symmetry of those images is not through the given dotted lines.