'Pond silk' is the common name of

The correct answer is option D. Spirogyra.

'Pond silk' is the common name given to Spirogyra, a filamentous green algae commonly found in freshwater habitats such as ponds, lakes, and slow-moving streams. Spirogyra is known for its long, thread-like filaments that appear silky and slimy.

Spongilla (option A) refers to a freshwater sponge, not 'pond silk'.
Chlorella (option B) is a genus of single-celled green algae that is not typically referred to as 'pond silk'.
Selaginella (option C) is a genus of spikemosses, which are vascular plants and not related to algae.

Therefore, the correct answer is option D. Spirogyra, as it matches the description of 'pond silk'.