Which example best demonstrates how unbalanced forces change the speed of an object's motion?

I don’t see a picture but unbalanced forces could be two boys pushing with a combined force of 400 Newton’s but the surface of what the box is laying on being 600 meaning since the ground is creating a higher force in the form of friction it will slow the boys down. When forces are unbalanced it means that the object can not be still or moving at a constant speed  when one force is greater by a significant amount the object either slows quickly or accelerates fast depending on which factor is greater.

A) A child on a bicycle braking at a stop sign.

Explanation:

A) A child on a bicycle braking at a stop sign.

it has to be b, about 97 percent.