Concept:Spirogyra is a green alga known for its ribbon-like, spiral chloroplasts and a central nucleus held by cytoplasmic strands.Explanation:Spirogyra cells contain one or more spiral-shaped chloroplasts winding along the cell wall.Its nucleus lies in the centre of the cell and is suspended by fine cytoplasmic strands.These features are unique to Spirogyra among the given options.Volvox forms colonial spheres, Paramecium is a ciliate protozoan, and Euglena has small chloroplasts without such spiral arrangement.Answer:B. Spirogyra