2. can cause a stationary object to start moving or a moving object to change its speed or direction or both.

As per Newton's law of inertia we can say every object will move in its state of motion either in the state of rest or will move with same constant velocity until some unbalanced external force will act upon it

so here we have to fill the space which says that stationary object can start or moving object will change its direction only in which case

so here the correct answer should be Unbalanced Force condition we will obtain such situation.

so we have

Unbalance force can cause a stationary object to start moving or a moving object to change its speed or direction or both.

diagram is missing, but i will try to explain your doubt.

answer:

amplitude is the maximum displacement or distance moved on a point of a vibrating body or wave measured from its equilibrium position.it is equal to half the length of vibration path.

the answer would be   dignified - apex