The correct answer is option 1, Spirogyra. Spirogyra is a filamentous green alga that is commonly found in freshwater ponds. It is known as `Pond Silk` because of its long, slimy, and hair-like appearance, which resembles silk. Spirogyra is a photosynthetic organism and forms green mats on the surface of the water. These mats can be easily seen as green patches or strands floating in the pond. It is an important part of the aquatic ecosystem as it provides food and habitat for various aquatic organisms. Additionally, Spirogyra has a spiral arrangement of chloroplasts, which are responsible for carrying out photosynthesis. Therefore, Spirogyra is commonly referred to as `Pond Silk` due to its slimy and silk-like appearance found in freshwater ponds.