Concept:Identify the organism that is both a nitrogen-fixing soil organism and a blue-green alga.Explanation:Blue-green algae are also known as cyanobacteria.Anabaena is a filamentous cyanobacterium common in soil and water.It fixes atmospheric N2 into usable forms.Rhizobium and Clostridium are bacteria, while Nitrosomonas is a nitrifying bacterium; none are blue-green algae.Thus, the correct answer is Anabaena.Answer:D. Anabaena