Because they lack well-defined and complex organ systems, we hypothesize that diverged from all other animals early in the evolution of animals.

poriferans
choanoflagellates
bilaterians
None of the answer options is correct.

Poriferans.

Explanation:

The evolution of the organisms has been occur from the simple organisms and leads to the evolution of the more complex organisms. This complexity arises due to the changing environment of the organisms.

The protozoans are the simplest organisms and all the other animal lineage has been derived from the porifera. The porifera are the cellular organisms that  do not have tissue level of organization and simple physiological system. Later, diverse and complex organisms arise from the porifera.

Thus, the correct answer is option (1).