Spirogyra is a
(a) Freshwater and free-floating algae
(b) Marine and free-floating algae
(c) Fresh water and locomotory algae
(d) None of the above

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Hint The organisms belong to the class Chlorophyceae also known as green algae are found floating on fresh water and some of them are found in backwater and saltwater. The algae commonly called blanket weed is found mostly in freshwater.

Complete answer:
Spirogyra belongs to the class Chlorophyceae and these members are commonly called green algae. The organisms of this class are unicellular, colonial, and filamentous. Due to the dominance of chlorophyll a and b, they are grass green in color. This pigment is localized in definite chloroplasts. They are mainly found in freshwater and also seen in backwater and saltwater. Spirogyra, Chlamydomonas, volvox, ulothrix, and chara. Spirogyra is free-floating algae found in freshwater.

Additional information:
- Algae, bryophytes, pteridophytes, gymnosperms, and angiosperms are placed under the kingdom Plantae.
- Chlorophyll-bearing, simple, thalloid, autotrophic, and largely aquatic (both freshwater and saltwater) organisms are algae.
- Algae is subdivided into three classes namely Chlorophyceae, Phaeophyceae, and Rhodophyceae.
- The organisms present in the Chlorophyceae are called green algae.
- Pyrenoids are the storage bodies (store proteins instead of fats) present in the chloroplast of green algae. They may be one or two.
- Spirogyra is a free-floating freshwater alga whose cell wall is made up of inner cellulose and outer pectose layer.
- They reproduce vegetatively by fragmentation, asexually by zoospores, and sexually by fusion of gametes.

So, the correct answer is ‘fresh water and free-floating algae’

Note: In Chlorophyceae, chlorophyll a and b are present, in Phaeophyceae chlorophyll a and c are present and in Rhodophyceae chlorophyll a and d are present.
Storage material in all the algae is starch but in the green alga, storage materials are proteins.