Answer:
D. (bottom right)
Explanation:
If there is a line where y = x, that means that y and x can both equal 1, 2, 3, -1, -2, -3, etc. They are just always the same. This line, then, would pass through (just for example) (-4, -4) and (4, 4). If you reflect the pink figure across that line, you will get the blue figure. Hope this helps!