In thallophytes, the female sex organ is a single-celled oogonium. It is not covered by any sterile jacket layer. In Bryophytes, the female sex organ is flask-shaped, multicellular archegonium which remains covered by a sterile jacket layer. Neck canal cells, ventral canal cell and egg are found within archegonium.

Thus, the correct answer is option D.

Oogonia of Thallophyta differs with archegonia of Bryophyta as follows:

1. Being unicellular and jacketless:
- Oogonia in Thallophyta are unicellular structures that develop directly from the haploid vegetative cells within the thallus.
- They lack a protective jacket or covering, making them exposed to the external environment.

2. Archegonia in Bryophyta:
- Archegonia in Bryophyta are multicellular structures that develop from the gametophyte.
- They are jacketed, meaning they have a protective layer called the jacket or venter, which surrounds the egg cell.

Explanation:
The oogonia of Thallophyta and archegonia of Bryophyta are both structures involved in the reproductive processes of these plant groups. However, they differ in their cellular organization, presence of a protective covering, and mode of development.

Oogonia of Thallophyta:
- Thallophyta is a division of plants that includes algae and fungi.
- In Thallophyta, the oogonia are unicellular structures that develop directly from the vegetative cells of the thallus.
- They are typically spherical or ovoid in shape and contain a single egg cell.
- Oogonia are not surrounded by a protective jacket or covering, which makes them vulnerable to environmental factors such as desiccation and predation.

Archegonia of Bryophyta:
- Bryophyta is a division of plants that includes mosses, liverworts, and hornworts.
- In Bryophyta, the archegonia are multicellular structures that develop from the gametophyte, which is the haploid phase of the plant's life cycle.
- Archegonia are jacketed structures, with a protective layer called the jacket or venter that surrounds the egg cell.
- The jacket consists of one or more layers of cells that provide protection and support to the developing egg cell.
- The neck canal cells present in the archegonium help in the passage of the sperm cells during fertilization.

Summary:
In summary, the main difference between the oogonia of Thallophyta and the archegonia of Bryophyta lies in their cellular organization and presence of a protective covering. Oogonia are unicellular and jacketless structures in Thallophyta, while archegonia are multicellular and jacketed structures in Bryophyta.