At what stage do stem cells start to lose their ability to differentiate into multiple cell types?

The correct choice is unipotent stem cells. Unipotent stem cells are characterized by their ability to differentiate into only one specific type of cell. This is a more restricted capability compared to their earlier stages.

During development, stem cells begin as totipotent, meaning they have the potential to become any cell type in the body, including extra-embryonic tissues. As development progresses, they can become pluripotent, which still allows them to differentiate into nearly all cell types, but they lose the ability to form all tissue types, particularly those necessary for forming a complete organism.

When stem cells transition to multipotent, they continue to have the capability to differentiate into several related cell types, though not as diverse as pluripotent cells. Finally, when stem cells reach the unipotent stage, they are limited to differentiating into a single cell type, marking the significant reduction in their differentiation potential. This reflects the process of differentiation as stem cells specialize and commit to specific lineages, ultimately losing their broader capabilities.