Cell aggregate body plan is found in the:

The correct option is C Porifera
Cell aggregate body plan is found in Porifera. Poriferans i.e., sponges have a cellular level of body organisation. In the cell aggregate body plan, the body consists of a cluster of or aggregation of cells that are not organized into tissues or organs. While Volvox is green algae and protozoan are non-phototropic unicellular eukaryotes.
Cnidarians have tissue grade of body organisation. Roundworms have organ system level of body organisation. Flatworms have organ and organ system level of body organisation.
So, the correct answer is porifera.