What type of force is acting in the cases given above?

(N/A) Accelerating unbalanced force: The velocity-time graph shows a constant positive slope,indicating uniform acceleration. According to Newton's second law,$F = ma$,an unbalanced force is required to produce acceleration.
$(b)$ No net force: The velocity-time graph is a horizontal line,indicating constant velocity (zero acceleration). According to Newton's first law,if the net external force is zero,an object will continue to move with a constant velocity.
$(c)$ Retarding unbalanced force: The velocity-time graph shows a constant negative slope,indicating uniform retardation (deceleration). An unbalanced force acting in the direction opposite to the motion is required to decrease the velocity.