The plants of which of the following plant kingdoms are known as algae ?

Thallophyte  

The correct answer is Option (4) – Thallophyte

The plants known as algae belong to the kingdom Thallophyta. Algae are a diverse group of photosynthetic organisms that can be found in various aquatic environments, including freshwater, marine habitats, and even damp terrestrial habitats. They range from single-celled microalgae to large, multicellular seaweeds. Algae are characterized by their lack of true roots, stems, or leaves, and they typically have simple body structures.

In the plant kingdom classification, Thallophyta represents the lowest and most primitive group, consisting of organisms with simple thallus-like structures. Algae are classified under this kingdom because they lack the complex structures found in higher plant groups like angiosperms (flowering plants), pteridophytes (vascular plants like ferns), and bryophytes (non-vascular plants like mosses). Instead, algae exhibit a wide range of forms, including unicellular, colonial, filamentous, and multicellular forms, making them diverse members of the Thallophyta kingdom.