The answer to this is A. Trenton was experiencing disequilibrium because he cannot comprehend why the sand that came from the other container seems to look more when placed in a skinny container.
He continued to do it and slowly, he had a realization about the sizes and the shapes of the containers. That is the moment when he had a realization and this is equilibrium. He has found some balance again because he already understood the reason why.
Usually, a sense of unbalance occurs whenever there are things that we do not understand. This does not only happen to kids. Adults can still have realizations every day in their lives.