The mirror image of along the dotted line is

The correct answer is option 4: 

• A vertical mirror (dotted line on the right) means left and right positions will interchange, while top and bottom remain the same.
• So:
  • The ‘+’ sign on the top-left shifts to top-right
  • The ‘+’ on bottom-right shifts to bottom-left
  • The filled circle (top-right) shifts to top-left
  • The empty circle (bottom-left) shifts to bottom-right
• The diagonal line also flips direction accordingly.

Only the last option correctly reflects all these changes.