Concept:Spirogyra is a filamentous alga whose body is formed from many cells arranged in a row.Explanation:Spirogyra consists of several cylindrical cells. These cells are linked end to end, forming a single long filament.Each cell has its own nucleus, cytoplasm, cell wall, and chloroplast. Because the filament is composed of many independent cells, it is considered multicellular.This end-to-end arrangement of cylindrical cells is the correct reason among the given options.Answer:A. the cylindrical cells are linked end to end