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 right diagonal in the question figure will appear as left diagonal in the mirror image, and thus the first and last options are eliminated.

Also, the small circle at top right will appear at bottom right in the mirror, so the second options is also not possible.

=> Ans - (C)