Answer: B
Explanation: A closed system can be defined as a system that does not allow for an exchange of matter with its surroundings.
An estuary where fresh water and ocean water mix is not a good example of a closed system because fresh water and ocean water mix out in the open and matter is exchanged.
A living single-called organism such as phytoplankton does not constitute as a closed system because in order for the phytoplankton to be considered alive it needs to interact with its surroundings i.e light from the sun so photosynthesis (converting light energy into chemical energy to be used by organism) can occur. This also happens out in the open
A river that flows into the ocean does not fit into the closed system definition. This is because a river that flows into the ocean allows for itself (matter) to mix with that of the ocean in an open space (rivermouth).
Therefore option B is the answer because it fits the definition. A chemical reaction within the deep ocean organisms that allows them to glow happens within (inside) the organism, making this system closed, hence there is no exchange of matter with surroundings.