Alternating current question

Alternating current:
a)always flows in the same direction
b)constantly changes direction
c)is necessary for the operation of a transformer
d) b & c

I think it is d but could anyone explain me how a transformer works with ac current? (I know they do but I don't understand why ac current is necessary for their operation.)

d) is correct

The magnetic lines of force in the primary coil expand and contract when AC voltage is applied, cutting through the coils of the secondary circuit and thereby inducing a voltage in the secondary.

If a DC voltage is applied to the primary, the magnetic field is stationary, and incapable of inducing a secondary voltage except when it is first applied and later when it is turned off (field collapses)

B and C is correct.
EDIT, if you connect DC to a transformer winding, it will produce a steady magnetic field in the core. As the current rises, a voltage will appear on the secondary, but as soon as the field is steady, that will disappear.
Because AC switches direction, the magnetic field is continually building up, dropping off, then changing polarity and building up again,before dropping off and repeating the cycle.
It is this continual changing of the magnetic field that will induce a changing and (again) AC voltage into the secondary winding(s) of the transformer.