If an animalia taxon ends in "ata":

it may be a phylum
it must be a phylum
it could be a phylum or class
it could be a phylum or order

In the kingdom Animalia, among all the phyla, orders, and class, only the phylum 'chordata' ends in 'ata'. The chordata includes the animals that have notochord present along wit the dorsal hollow nerve cord. This phylum includes the deuterosomes that indicates that the anus is formed before the mouth. Since, this is the only taxon in Animalia ending with 'ata', it is known that such a taxon will surely be a phylum.

Hence, the correct answer is 'it must be a Phylum'.

they deliver oxygen to the blood