What type of cells can give rise to a new organism?

Totipotent cells are unique in that they possess the capability to develop into a full organism. These cells can differentiate into all types of cells necessary for the formation of an entire organism, including both the embryonic and extra-embryonic tissues, such as the placenta. Totipotent cells are typically found in the earliest stages of development, such as the fertilized egg (zygote), which is capable of developing into a complete organism.

In contrast, while embryonic stem cells (the first option) are pluripotent and can give rise to many cell types, they cannot form an entire organism on their own as they typically can only differentiate into cell types of the three germ layers: ectoderm, mesoderm, and endoderm. Specialized cells (the second option) have already differentiated to perform specific functions and cannot revert back to form a whole organism. Functional proteins (the last option) are not cells at all; rather, they are the products of gene expression that perform various roles in cellular functions, but they do not have the capability to develop into an organism.

Thus, the correct answer highlights the special quality of totipotent cells in their potential to develop into a complete, independent organism.