Totipotent stem cells are unique as they have a greater developmental potential compared with other stem cells. Having the ability to isolate and culture totipotent stem cells creates many therapeutic and research possibilities related to:

Studying zygotic genome activation (the point at which development becomes exclusively controlled by the zygotic genome, rather than the maternal genome)

Rewiring the epigenome (treating pathological conditions induced by epigenetic alterations)

Understanding early embryonic development in more detail

Creating human-animal chimeras (theoretically, animals with human organs could aid disease modeling, drug development and transplantation)

Stem cells are increasingly being used as model systems in research.

Explanation:
Definition of stem cells:
- Stem cells are a type of cells that have the ability to differentiate into different cell types and self-renew.
- They are found in various tissues of the body and play a crucial role in growth, development, and repair.
Types of stem cells:
There are different types of stem cells based on their potential and origin. In the context of the given question, the correct answer is d. Totipotent cell. However, it is important to mention the other types of stem cells as well.
1. Totipotent cells:
- These are the most potent type of stem cells.
- They have the ability to differentiate into any cell type, including both embryonic and extraembryonic cells.
- They can give rise to an entire organism, including the placenta and other supporting tissues.
2. Pluripotent cells:
- These stem cells are derived from the inner cell mass of the blastocyst (an early-stage embryo).
- They can differentiate into any cell type of the body, but not the extraembryonic cells.
- Examples include embryonic stem cells and induced pluripotent stem cells.
3. Multipotent cells:
- These stem cells can differentiate into a limited number of cell types.
- They are found in specific tissues of the body, such as bone marrow, blood, and skin.
- Examples include hematopoietic stem cells and mesenchymal stem cells.
4. Unipotent cells:
- These stem cells have the ability to differentiate into only one specific cell type.
- They are usually found in adult tissues and play a role in tissue repair and regeneration.
- Examples include muscle satellite cells and basal cells of the skin.
Stem cells used in medical sciences:
- Stem cells, especially pluripotent and multipotent cells, are extensively used in medical sciences for various purposes.
- They have the potential to be used in regenerative medicine, tissue engineering, and disease modeling.
- Stem cell-based therapies are being explored for the treatment of diseases such as cancer, heart disease, diabetes, and neurodegenerative disorders.
- These therapies aim to replace damaged or dysfunctional cells/tissues with healthy functional ones derived from stem cells.
In conclusion, stem cells are undifferentiated cells that have the ability to differentiate into different cell types and self-renew. The correct answer to the given question is d. Totipotent cell, which refers to the most potent type of stem cells that can differentiate into any cell type, including both embryonic and extraembryonic cells.