Spirogyra belongs to which of the following classes?

The correct option is Chlorophyceae.

Explanation:
Spirogyra is a multicellular, filamentous green alga that belongs to the class Chlorophyceae (under the division Chlorophyta). Members of this class are commonly referred to as green algae because of the dominance of pigments chlorophyll a and chlorophyll b, which give them a characteristic grass-green color. Spirogyra is particularly known for its unique spiral or helical arrangement of chloroplasts within its cells.

Why other options are incorrect:
- Cyanophyceae: This class consists of blue-green algae (cyanobacteria), which are prokaryotic photosynthetic organisms.
- Phaeophyceae: This class comprises brown algae, which primarily contain chlorophyll a, chlorophyll c, and fucoxanthin (giving them a brown color).
- Rhodophyceae: This class comprises red algae, which contain pigments like phycoerythrin that give them a red appearance.