An unbalanced force is at work when a. a book sits on a shelf b. a piece of paper is on the floor c, a glass sits on a table d. a model rocket takes offs

There is no such thing as "an unbalanced force".

The group of all forces acting on an object is either balanced or unbalanced.

-- If the forces acting on an object are unbalanced, then the object is accelerating ... its speed or direction is changing.

-- If the forces acting on an object are balanced, then the object is not accelerating ... its speed and direction are not changing.

-- If neither the speed nor the direction of an object is changing, then the forces on the object are balanced.

-- If either the speed or the direction of an object is changing, then the forces on the object are unbalanced.

-- Neither the speed nor the direction of a book on the shelf, a paper on the floor, or a glass on the table are changing.  The forces on these objects are balanced. (A, B, and C)

-- The speed of a model rocket is changing.  The forces on the model rocket are unbalanced. (D)

the body's situation won't change the forces are equal opposit and on the same body

increase masses, decrease separation