The correct options are:
A. change the direction of its velocity.
B. change the magnitude of its velocity.
For instance, A is true in case of a uniform circular motion: if there is an unbalanced force that acts as centripetal force, this force changes the direction of the velocity of the object (it makes it turning in a circular path) but not its magnitude.
Instead, B is true for instance when an unbalanced force F is applied in the same direction of motion of the object: the object will experience an acceleration given by F=ma, and so since the object accelerates, the magnitude of its velocity will change.