Correct Answer: a-iv, b-iii, c-ii, d-i

Solution : The correct option is a-iv, b-iii, c-ii, d-i .

Blue-green algae, or cyanobacteria, are photosynthetic bacteria that often form slimy colonies. They play a crucial role in nitrogen fixation and oxygen production.

Red algae, scientifically known as Rhodophyta, are predominantly marine and appear red due to the presence of pigments like phycoerythrin.

Green algae, exemplified by Chlamydomonas, are a diverse group of photosynthetic organisms. They share characteristics with land plants and contribute significantly to aquatic ecosystems by producing oxygen through photosynthesis.

Brown algae, such as Sargassum, are multicellular seaweeds found in marine environments. They are characterised by their brown colouration due to the presence of pigments like fucoxanthin.