Answer:
Option b. Dark spot
Step-by-step explanation:
Michelson interferometer gives the path difference of the light as path difference, d as twice the distance moved or covered by the mirror
, x and is given as:
d= 2x
Since, its an even multiple, we obtain a bright fringe
now, when the move half the distance, i.e.,

Therefore, the path difference for half the distance
is:
d = x
As it is clear that its an odd multiple which correspond to dark spot as the final image