Spirogyra is commonly called pond silk because


 

Concept: Pond silk is the common name of Spirogyra (algae) because it is very slimy.
Spirogyra is a genus (about 400 species) of freshwater green algae which are found in shallow ponds, ditches and amongst vegetation at the edges of large lakes, generally growing free-floating.
Hence, option 3 is correct.