The correct answer is: option (iii)

Since a plane mirror is being used in the above problem and a plane mirror the distance between the object and mirror is equal to the distance between image formed and the mirror.

Distance between mirror and image of David = 4m

When David moved 1m towards the mirror, then distance becomes = 3m

Distance of object from the mirror = distance of image from the mirror = 3m

Distance between David and his image = 3m +3m = 6m