The correct answer is
speed
In fact, Newton's second law states that:
[tex]\sum F = ma[/tex]
where the term on the left is the resultant of the forces acting on an object, m is the object's mass, and a is its acceleration. We see from the formula that if forces acting on an object are unbalanced, this will result in a non-zero acceleration of the object.
A non-zero acceleration means a change in velocity of the object: since velocity is a vector, it consists of a magnitude (the speed) and a direction. Therefore, a non-zero acceleration means a change in either the speed of the object, or in its direction.
