Plant body is thallus in
(a) Gymnosperms
(b) Algae
(c) Bryophyta
(d) Pteridophyta

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Hint: Thallus is the entire body of a multicellular organism that is non-moving and has no organization of tissues into the organs. they do not consist of distinct parts such as leaves, roots, and stems and consist of vascular plants that have analogous structures.

Complete answer:
The plant body is thallus in Algae. Thallus constitutes filaments or cells which range in size from a unicellular structure to a complex structure. Algae consist of photosynthetic eukaryotic organisms which include multiple species of distinct clades. They range from unicellular microalgae like chlorella to the diatomaceous multicellular forms such as the giant kelp which is a brown alga and can grow up to 50 meters. They consist of a mostly autotrophic and aquatic organism that lacks distinct cell and tissue types. The most complex Marine algae and the largest Algae are known as seaweeds and the most complex freshwater algae are the charophyte which is the division of green algae that includes spirogyra and stoneworts. They do not include common ancestors and consist of a single origin from cyanobacteria. They exhibit reproductive strategies from asexual cell division to the complex forms of sexual reproduction. They contain chloroplast which contains circular DNA and represents reduced endosymbiotic cyanobacteria.

Additional information:
Gymnosperms: They are a group of seed-producing plants that develop on the surface of scales or leaves. The gate is modified to form cones or ginkgo. Gymnosperms are divided into six phyla. They are paraphyletic and include many distinctive taxa in their fossil record which do not belong to the modern groups that include seed-bearing trees having fern-like vegetative morphology. They originated in the late Carboniferous period which replaces the lycopsida rainforest of the tropical region and is a result of whole-genome duplication around 319 million years ago.
Bryophyta: They are non-vascular land plants that consist of about 20,000 plant species and do not produce flowers or seed. Reproduction in bryophytes takes place through spores and is considered to be a paraphyletic group. Sporophytes produced are unbranched and the lifecycle gets dominated by the gametophyte stage. They do not have true vascular tissue which contains lignin which helps in the transport of water.
Pteridophytes: They are vascular plants that disperse spores because they do not produce flowers and seeds and are known as cryptogams. They do not form a monophyletic group and are closely related to the seed plants. The sporophyte body is differentiated into root, stem, and leaves and the root system is adventitious. Pteridophytes share common ancestors and do not form a clade but constitute a paraphyletic group. The undergoes alternation of generation in which a diploid generation is followed by a haploid generation that produces gametes.
So, the correct answer is '(b) Algae'.

Note: Kelp is the largest algae that can grow up to 200 meters and is brown algae. The largest bryophyte is Dawsonia and the smallest bryophyte is Zoopsis. Fern is a pteridophyte with 10560 known species and some of them constitute the oldest plant in the world. They can live for about a hundred years and do not produce seeds.