Here's why:

* Algae are photosynthetic organisms, meaning they produce their own food through sunlight. Spyrogyra has chloroplasts, which contain chlorophyll for photosynthesis.

* Bacteria are single-celled prokaryotes (lacking a nucleus and other membrane-bound organelles). Spyrogyra is a multicellular organism with a complex structure.

* Protozoa are single-celled eukaryotes (have a nucleus and other membrane-bound organelles). While Spyrogyra is unicellular in its initial stages, it forms long filaments of connected cells, making it multicellular.

So, Spyrogyra is classified as a member of the Kingdom Plantae and specifically belongs to the algae group.