Question

which of the following organism is considered to be the most important organism in an ecosystem?
tertiary consumer
realized species
keystone species

Explanation:

Tertiary consumers are top predators but not the most critical to ecosystem structure. "Realized species" is not a standard term for this context. Keystone species have an outsized, critical role in maintaining ecosystem balance; their removal causes drastic ecosystem changes, making them the most important organism type here.

Answer:

keystone species