Which one of these is not a eukaryote ?

Solution: In above question, Anabaena is the only organism that is not an eukaryote and only possess prokaryotic characteristic features. i.e., absence of membrane bound organelles and undefined nucleus.
Whereas, Euglena, Spirogyra and Agaricus are eukaryotes, possessing membrane bound organelles as mitochondria and nucleus (well defined).