Final answer:
The common ancestor of all animals is most likely a B. colonial, flagellated protist, given the similarities between sponge choanocytes and choanoflagellates.
Step-by-step explanation:
The common ancestor of all animals is likely to have been a colonial, flagellated protist. This assertion is based on the similarities between the feeding cells of sponges, known as choanocytes, and colonial flagellated protists known as choanoflagellates. It is believed that these choanoflagellates may resemble the common ancestor of sponges, and potentially all animals due to their unique characteristics such as having a single posterior flagellum and a collared cell for feeding, much like sponge choanocytes. Hence, the answer to the question is B. colonial, flagellated protist.
The common ancestor of all animals is likely to have been a colonial, flagellated protist. Sponges, which are colonial, flagellated protists, are considered to represent an early stage of multicellularity in the animal clade. Choanoflagellates, which are animal-like protists, resemble the common ancestor of sponges and all animals. They have a single, apical flagellum surrounded by a contractile collar, similar to the feeding mechanism seen in collar cells of sponges.