Agree- The Statement above it’s true in many ways, while Unicellular organisms can be complex, they are simply a one called organism. Multicellular organisms are organisms that consist of more than one cell and life forms making them more diverse in complex.
Evidence-
Complex organisms are multicellular with specialized tissues. That is tissues like nerves, muscles, skin etc that do a specific job. Humans are multicellular organisms with multiple types of specialized tissues so we are complex organisms. Complex Organisms are life forms made of other life forms so overall a one called organism is not as complex.