(D) The two-kingdom system of classification was accepted for a long time,but it had several limitations:
$(i)$ It failed to distinguish between prokaryotes (like bacteria) and eukaryotes.
$(ii)$ It did not differentiate between unicellular and multicellular organisms.
$(iii)$ It failed to distinguish between photosynthetic (green algae) and non-photosynthetic (fungi) organisms.
$(iv)$ Fungi have chitinous cell walls,which are chemically different from the cellulosic cell walls of plants.
$(v)$ Organisms like Euglena possess both plant-like (photosynthetic) and animal-like (heterotrophic) characteristics,making their placement difficult.
$(vi)$ Lichens,which are symbiotic associations of algae and fungi,could not be classified into either kingdom.
$(vii)$ Viruses,which lack cellular machinery and protoplasm,do not fit into this system as they are neither truly living nor non-living.