The correct option is A Monera
Blue-green algae are members of monera. They are cyanobacteria. They are Gram positive photosynthetic prokaryotes which perform oxygenic photosynthesis. They are one of the most successful autotrophic organisms on earth which have mastered all types of environments- fresh water, sea water, salt marshes, moist rocks, tree trunks, moist soils, hot springs, frozen waters. Blue-green algae may be unicellular, colonial or filamentous. Cells are larger and more elaborate than bacteria. Heterocyst is present for nitrogen fixation.