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

A horizontal mirror is placed, so the objects will turn upside down in the mirror image (eg - an object at top right will appear at bottom right). So, the two circular rings at the left diagonal will appear at the right diagonal, and thus the third option is eliminated.

Also, the smaller circle at bottom left will appear at top left and the line inside will be reversed, so the first two options are not possible.

=> Ans - (D)