Why were Chlamydomonas and Chlorella placed in Kingdom Protista under the five-kingdom classification?
The five-kingdom system places single-celled eukaryotic organisms in Kingdom Protista. Thus, Chlamydomonas and Chlorella, although previously grouped with plants and possessing cell walls, are included in Protista.
Ref: NCERT Class 11 Biology Chapter 2: Biological Classification Five Kingdom Classification and Kingdom Plantae Animalia