An object is moving at 5 m/s and a few seconds later it is moving at 3 m/s. what had to have happened? a. the object's net force was 0 n. b. the object's acceleration increased. c. an unbalanced force acted on the object

The correct option is C.
From the question given above, it can be seen that the acceleration of the moving object has been reduced from 5 m/s to 3 m/s, this happened because an unbalanced force acted on the object and slowed it down. An unbalanced force is any force that causes a change in motion of a moving object.

Milliseconds. . . . . .