Answer:
B
Explanation:
Well it is certainly not no effect. Something has to happen. Not D.
If the balances are unbalanced it means that there is a force left over that can cause an acceleration. The speed is not constant when there is an acceleration. Not C
Unbalanced forces mean that there has to be something happening. Unless the friction cannot be overcome, there will be motion. Not A. But it is your second best answer if there is friction.
The answer I think you are intended to give is B.