The correct answer is option D, that is, stem cells.
The stem cells comes under the category of undifferentiated cells, which possess the tendency to divide into unique kinds of cells. The human embryonic stem cell are regarded as immortal, as they do not age, they can multiply indeterminately, and produce any tissue of the organism. As such, they do not amass destructed proteins like the ones associated with the disorders like Huntington's or Alzheimer's.