Concept:Multicellular organisms are made up of many cells, whereas unicellular organisms consist of a single cell.Explanation:Spirogyra is a filamentous alga composed of many cells arranged end to end, making it multicellular.Chlamydomonas is a single-celled green alga.Amoeba is a single-celled protozoan.Euglena is a single-celled flagellated organism.Therefore, only Spirogyra is multicellular among the options.Answer:B. Spirogyra