Option D is the correct answer. This is because, Spirogyra exist as filament that occurs in rows or chains. Besides being free-living, organisms can also exist as colonies, filaments, or in a multicellular state.

Spirogyra exists as filaments. It's a type of green algae that grows in long, unbranched chains of cells, forming these thread-like structures. These filaments are commonly found in freshwater habitats like lakes and ponds, often referred to as "water silk" or "pond silk".