In which situation is the object experiencing unbalanced forces?

a box resting on a horizontal floor

a car slowing as it reaches a stop light

a car with its cruise control set to 50 mph

a rocket sitting on the launch pad

The moving car experiences unbalanced forces when applying brakes; when a car is slowing as it reaches a stop light.

When the car driver applies brakes to a car in order to stop, an unbalanced force is applied to the car. The speed of car gradually reduces and there’s a decrease in acceleration; a net force acts on it.

Options A and D has a box and a rocket always in a rest position. A car with cruise control is an adaptable speed setting for a vehicle which helps it to maintain a fairly constant speed. Constant velocity gives zero acceleration and hence no considerable unbalanced forces appear.