Final answer:
The first known genus in green algae to evolve a filamentous form is Ulothrix, representing a key transitional form in algal evolution towards more complex structures.
Step-by-step explanation:
The first known genus in green algae to evolve a filamentous form is Ulothrix. Ulothrix is a genus of filamentous green algae that exhibits a string of similar-looking cells, signifying an early step towards complexity in green algal evolution. Unlike some other genera such as Ulva, which forms multicellular sheets, Oedogonium which is another filamentous green algae, or Chlamydomonas which consist of single-celled organisms, Ulothrix represents an important transitional form in the adaptation of a filamentous structure.