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 (e.g - an object at top right will appear at bottom right). So, the left diagonal will appear as right diagonal in the mirror image, and thus the last two options are eliminated.

Also, the bigger circle will have horizontal diagonal and smaller one will have vertical, so the second option is also not possible.

=> Ans - (A)