Final answer:
The protist with a calcium carbonate shell that uses pseudopodia for movement is likely related to foraminiferans in the Rhizaria supergroup.
Step-by-step explanation:
A marine biologist analyzing water samples notices a protist with a calcium carbonate shell that moves by pseudopodia extension. The protist described is likely to be closely related to species within the group known as foraminiferans, which are part of the Rhizaria supergroup. This group is known for having protists with elaborate tests (shells) made of calcium carbonate and for using pseudopodia for movement and feeding.