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The x,y co-ordinates of all the points will be reversed after reflection across line l. D is the only point which co-incides with the reflected co-ordinates. Its co-ordinates before reflection are (4,9) (assuming each gradient to be of 1 unit), and after reflection are (9,4) which co-incides with point D.
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As you can see in the attached image, I have marked all vertices as A,B,C,D,E and then compared their x,y co-ordinates witheir counterparts after reflection. Only D has its co-ordinates matching as reversed post reflection around the y=x line.
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Reflection.png
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Reflection.png
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So for reflection, coordinates are reversed ? Thank you for posting the image.

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So for reflection, coordinates are reversed ? Thank you for posting the image.

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its reversed because it is being reflected across y=x line in Q1. for a different line acting as reflector, it will be slightly different. for example - try doing the same problem in Q2 across line y=-x
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