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)

it would be kevlar ^^