In young's double slit interference experiment the incident light consists of 2 wavelenghts
1 &
2 & the slit separation d & D is the distance between the slits & the screen.The maxima due to the two wavelenghts will coincide at a dist from the central maximum which is
1+
2/2(D/d)
1 -
2/2(D/d)
Least common multiple of
1 D/d and
2D/d
Highest common multiple of
1 D/d and
2D/d
ans is c