Mcrit is the lowest free stream mach number at which we get sonic (Mach 1) airflow at any point on the surface of the aircraft. For the purposes of this type of question the examiners usually we assume that this will occur over the wings.
This means that Mcrit = Mach 1 minus the airflow acceleration over the wings.
So
Anything that increases this acceleration will decrease Mcrit
Anything that decreases this acceleration will increase Mcrit.
If we decrease wing area while keeping aircraft weight unchanged, then each square foot of wing will need to produce more lift. This will require a greater angle of attack, which will increase acceleration over the wing. This means that airspeed over the wing will be closer to Mach 1 so Mcrit will be reduced. So option A is incorrect.
The wings must produce sufficient lift to carry the weight of the aircraft plus any downward force from the tail plane. If we move the CG aft we need less tail plane down force, so we need less lift. This enables us to use a lower angle of attack, so Mcrit is increased. Option d is the correct answer.