Final answer:
Plants are part of the Archaeplastida supergroup and are most closely related to charophyte green algae, due to their shared structural and biochemical traits.
Step-by-step explanation:
Plants are members of the eukaryotic supergroup Archaeplastida, and their closest relatives are charophyte green algae.The classification and evolutionary relationships of plants within the eukaryotic domain are in ongoing areas of research. The green algae, comprising Chlorophyta and Charophyta, share a number of characteristics with land plants, such as photosynthetic ability, chlorophyll a and b, starch as storage carbohydrate, and cellulose cell walls. While chlorophytes are related to land plants, it is the charophytes that are the closest relatives.
This is supported by various structural and biochemical similarities, including mechanisms of cell division and shared biochemical pathways that are not seen in other algae. Specifically, groups like the Charales, Coleochaetales, and Zygnematales within the charophytes present traits that closely resemble those of early land plants, such as the formation of sporopollenin and precursor compounds to lignin. The Archaeplastida supergroup encompasses land plants and both chlorophyte and charophyte green algae, but it is with the charophytes that land plants share the most direct lineage. Therefore, considering the phylogenetic evidence, the designation of plants and their closest relatives can be more accurately reflected.