Nostoc form

The correct option is both (a) and (b).

Nostoc is a genus of cyanobacteria (blue-green algae) known for forming colonies. The structure and arrangement of these colonies can vary depending on their developmental stage and environmental conditions:
1. Simple Colony: In the early stages of development or under certain conditions, Nostoc can exist as simple filaments (trichomes) enclosed in a gelatinous sheath, forming basic or simple colonial aggregates.
2. Compound Colony: As the colony grows and matures, multiple filaments intertwine and aggregate together within a common, larger mucilaginous matrix, forming complex, macroscopic structures known as compound colonies. These compound colonies can often be seen with the naked eye as ball-like or gelatinous masses on damp soil or floating in water.

Therefore, since Nostoc can exhibit both forms, the correct choice is both (a) and (b).