Imagine that you are observing an enzyme-catalyzed reaction in lab. every time you add more enzyme, the reaction rate increases proportionally until the reaction rate suddenly levels off. no other chemicals were added, and no modifications were made in the experimental setting. why do you think the reaction rate stopped increasing?

Enzymes catalyze the chemical reactions, they act upon the reaction substrates and speed up the reaction. Enzymes have active sites, the places where the reaction substrates interact with the enzyme bringing about the conversion of substrates to products. So, as the enzyme concentration increases the rate of reaction increases till a point where the rate is leveled off. The rate does not further increase, as the substrate might have become limiting at that point. All the available amount of substrate would have been associated with the active sites of the enzymes. So, at that point although there is enough catalyst, lack of substrate would limit the rate of reaction.

geosphere and hydrosphere.

this is because the geosphere has to do with geological events, while the hydrosphere controls all water on earth.