Answer:
A
Step-by-step explanation:
An organism is an individual living thing.
One fish
A population is a group of organisms of the same species living in the same area.
A school of fish
A community is many species living in the same area.
Fish, ducks, grass, and plants
An ecosystem is all the living things and nonliving things in the same area.
Water, air, rocks (abiotic-nonliving), fish, ducks, grass, plants (biotic-living)
Therefore, from least to most complex, it is:
organism-->population-->community-->ecosystem