Concept:Free-living organisms are independent, usually single-celled organisms that do not occur as attached colonies or filaments.
Explanation:Amoeba,
Chlamydomonas, and
Euglena exist as solitary, free-living cells.
They carry out all life activities independently in their environment.
Spirogyra, however, is a filamentous green alga.
Its cells are arranged in end-to-end rows or chains, forming a visible filament.
Because it exists as a filament rather than as a single free-living organism, it is the exception.
Answer:D.
Spirogyra