To solve this kind of question, you have to do in stages.
1) Consider the object O and the mirror at the top. The image formed will be equal distance perpendicular to the mirror (follows the 5 characteristics of mirror image). Let the image be O’.
2) Now treat O’ as the new object and consider the mirror at the bottom. Likewise apply the same principle, the image of O’ is now at position D. Alternatively, a fast and simple way is based on the concept that to our eyes, light seems to travel in straight line. Hence to our eyes, the image of O is directly in front.