See the picture in the white box. On which of the dotted lines will you keep the mirror so that you will get the shape (b)? 

Comparing with the image in the white box and the image (b), we can say that clearly image (b) has the left side of the image in white box.
Thus, if we keep a mirror in the dotted line 1, the image on the left side of dotted line 1 will get reflected in the mirror to obtain the image (b).
Therefore, option A is the correct answer.