The explanation that best captures the essence of Newton's Third Law of Motion in the context of a fish swimming through water is the first one:
"A fish pushes water backwards with its fins (action). The water pushing the fish forward is the reaction."
This correctly describes the interaction between the fish and the water. According to Newton's Third Law, for every action, there is an equal and opposite reaction. When the fish uses its fins to push against the water, it creates a force that moves the water backwards. In response, the water exerts an equal and opposite force that propels the fish forward.
While the other options discuss aspects of fish movement, they do not explicitly frame the interaction in terms of action and reaction as clearly as the first option does.