The correct response is:
When the octopus ejects the water out, the water applies an equal force on the octopus in the opposite direction.
This illustrates Newton's Third Law, which states that for every action, there is an equal and opposite reaction. In this case, the action is the octopus ejecting water out of its funnel, and the reaction is the equal force that the expelled water exerts back on the octopus, propelling it in the opposite direction.