My best guess would be:
"A force equal in magnitude but opposite in direction"
However I assume that this question is multiple choice, by the way it is introduced. Therefore it would be helpful if these options were also displayed - hence take this as my best guess only.
Answer:
Decrease the charge of one of the particles
Answer:
1. The reason why a plane mirror forms a virtual image is because it doesn't let the light go through it, but it reflects the light, so if you go behind the plane mirror, then you won't see the rays of light since it's not letting the rays of light go behind it.
2. The image cannot be projected onto a screen also.
3. Also, the Rays of light never meet at a focus point, so hence, this is why the plane mirror forms a virtual image.
Thanks!
