Concept:The group of animals that completely lacks a nervous system is the simplest multicellular phylum, Porifera.
Explanation:Porifera, commonly called sponges, are multicellular organisms but do not possess specialized nerve cells or any nervous tissue.
They respond to stimuli only through simple contractions of their body.
In contrast, Cnidaria have a nerve net, Platyhelminthes possess nerve cords and ganglia, and Nematoda also have a well-defined nervous system.
Hence, among the given options, only Porifera are without a nervous system.
Answer:A. Porifera