a). balanced force
b). unbalanced force
There's no such thing as either of these. A group of two or more forces can be balanced or unbalanced. A single force can't be.
c). gravitational force ... doesn't cause an object to move in a circle;
Drop a stone from the roof of a tall building and watch it fall.
It goes straight down, not in a circle.
d). centripetal force ... force directed toward the center of a circle,
causes an object to move in a circle.
