True nucleus is absent in Vaucheria, Volvox, Anabaena, Mucor

Vaucheria is a yellow-green alga belonging to family Vaucheriaceae under the kingdom Plantae.
Volvox is a green alga belonging to family Volvocaceae under the kingdom Plantae.
Anabaena is a blue-green alga belonging to phylum Cyanobacteria under the kingdom Monera
Mucor is a fungus belonging to phylum Zygomycota under the kingdom Fungi.
Vaucheria, Volvox, and Mucor have domain Eukaryota while Anabaena has domain Prokaryota. Eukaryotes have membrane-bound well-defined nucleus whereas, Prokaryotes do not have a membrane-bound nucleus because, in them, the genetic material lies freely in the cytoplasm. Hence, the true nucleus is absent in Anabaena.
So, the correct answer is 'Anabaena'.