Question
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First action spectrum of photosynthesis proposed based on a study
(a)Chlorella
(b)Stellaria media
(c)Oscillatoria
(d)Cladophora

Answer
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Hint: It belongs to the group of algae having reticulate filaments as their characteristic features. They have both the gametophytic and the sporophytic life cycles. They are photosynthetic in nature.

Complete answer:
The first action spectrum of photosynthesis proposed based on a study in Cladophora. The rate of the biological effectiveness which is plotted in a graph against the wavelength of light is known as the action spectrum which is related to the absorption spectrum in certain ways. The level of biological activity is difficult to describe as the responses are nonlinear with intensity. Certain species of filamentous algae is Ciliophora which is hard and comprises the great variation in its appearances. The two life cycles consist of haploid gametophyte and diploid sporophyte which are the multicellular stages that look highly similar. They can be distinguished by counting the chromosomes or by examining their offsprings. Haploid gametes are produced by haploid gametophyte by the process of mitosis and haploid spores are produced by a diploid sporophyte by the process of meiosis.

Additional information:
Chlorella: They constitute single-celled green algae which are about 2-10 micrometers in diameter and have no flagella. It is high in protein and is therefore used as a food source that is used to cultivate mass production of man-made circular ponds. It is found in certain liquid-based cocktails as an ingredient. In various regions of the United States and Canada, it is consumed as a health supplement and a food supplement in Japan.
Stellaria media: It is an annual plant that is a perennial flowering plant and is used as a cooling herbal remedy. It is grown as a vegetable crop for both human and poultry consumption and is known as common chickweed. It germinates in autumn or late winter and produces large mats of foliage. Some plant species constitute toxins which are known as saponins and are consumed in large quantities by some species. It is used as a folk medicine due to its medicinal properties which are used to treat itchy skin conditions and pulmonary diseases.
Oscillatoria: It is a filamentous cyanobacterium that occurs in freshwater ponds. It uses photosynthesis and fragmentation to survive and reproduce.
So, the correct answer is option (D) 'Cladophora'.

Note: Chlorella constitutes 30 different species some of which are most commonly used in research. Humans are unable to digest chlorella due to its hard cell wall and can take it as a supplement to reap its benefits. Chickweed constitutes 25 species and is a herb that is grown under the shade of oak trees.