Final answer:
The totipotent stem cell is called an archaeocyte (A), which has the ability to differentiate into any cell type necessary for an organism's development.
Step-by-step explanation:
The totipotent stem cell is called an archaeocyte (A). Totipotent stem cells are the most versatile cells, with the capability to differentiate into any cell type required for an organism's development. This characteristic is what makes the very first embryonic cells so powerful, as they give rise to all the various specialized cells that make up an organism.
In contrast, other types of specialized cells in sponges, like choanocytes, sclerocytes, and pinacocytes, serve specific functions but do not have the totipotent capacity that archaeocytes possess.