In which situation is the object experiencing unbalanced forces?
a. a box resting on a horizontal floor.
b. a car slowing as it reaches a traffic light.
c. a car with its cruise control set to 30 km/h.
d. a penny at terminal velocity after falling off of a building.

b).  Since the car's speed is changing, there is a net force acting on it,
so the forces on it are unbalanced.

c).  Setting the cruise control doesn't mean that the car absolutely positively
maintains that exact speed at every instant. 

What the cruise control actually does is:

-- When the car's speed drops a little bit, the cruise control gives it
a little more gas, to bring the speed up to where it belongs.

-- When the car's speed increases a little bit, the cruise control lets
off on the gas a little bit, to let the speed drop to where it belongs.

These small increases and decreases of the car's speed are happening
all the time when the cruise control is engaged.  So you have to say that
the net force on the car is seldom zero, and the forces on it are constantly
unbalanced, either one way or the other.

Because the dept in the ocean