C. The net force is perpendicular to the motion.
When you swing a yo-yo in a circle around your head, the net force
is the tension in the string, pointing toward the center of the circle.
There's nothing there that provides any force parallel or opposite to
the motion.
When the Earth revolves around the Sun in a circular orbit, the net force
is the gravitational force between the Sun and the Earth. There's nothing
there that provides any force parallel or opposite to the motion.
