Final answer:
An object remains at rest or in motion unless acted upon by an unbalanced force, in reference to Newton's First Law of Motion, or law of inertia. The correct answer is B. Unbalanced, as an unbalanced force is needed to change the state of motion of an object.
Step-by-step explanation:
The principle you are referring to is Newton's First Law of Motion, often called the law of inertia. This law states that an object in motion will stay in motion, and an object at rest will stay at rest unless acted upon by an unbalanced force. Therefore, the correct answer to your question is B. Unbalanced. An unbalanced force is necessary to change the motion of an object - either to start moving an object that is at rest or to change the velocity (speed or direction) of an object in motion.
For example, if you push a book across a table, the unbalanced force provided by your hand causes the book to move. However, once you stop pushing, friction - an unbalanced force - will cause the book to come to a stop.
The inertial properties of an object, which describe how much it resists change in motion, are directly related to its mass. More massive objects require larger unbalanced forces to change their motion compared to less massive objects.