The correct answer is
b.the nut experiences unbalanced forces and accelerates.
In fact, the nut experiences basically one force, the gravity, directed downward (there is also the air resistance, but its intensity is negligible compared to the gravity). So, the gravity is not balanced by other forces, and so the nut experiences an acceleration directed downward, due to Newton's second law:
F=ma
which states that an object of mass m (the nut) that experiences unbalanced forces, accelerates, and a is the value of its acceleration.
