The plant which has changed very little during the last several million years is Maiden hair tree.

Maiden Hair Tree:
- The Maiden Hair Tree, also known as Ginkgo biloba, is a unique plant species that has remained unchanged for millions of years.
- It is considered a living fossil because it is the only surviving member of the Ginkgoaceae family, which dates back to the Permian period, about 270 million years ago.
- It is native to China and is widely cultivated for its medicinal properties and ornamental value.
- The Ginkgo tree has a distinct fan-shaped leaf and can live for over a thousand years.
- It is also known for its remarkable resistance to pests, diseases, and environmental stressors.

Maiden Hair Fern:
- The Maiden Hair Fern, on the other hand, is a different plant species that belongs to the Adiantaceae family.
- It is a delicate, lacy fern that is commonly used as a houseplant or in ornamental gardens.
- Unlike the Ginkgo tree, the Maiden Hair Fern has evolved over time and has undergone several changes in its morphology and genetics.

Sanjeevani Buti:
- Sanjeevani Buti is a mythical herb that is believed to have medicinal properties and is mentioned in Hindu mythology.
- It has not been scientifically identified and is not recognized as a plant species.

Sago Palm and Sargasso:
- Sago Palm and Sargasso are two other plant species that have also evolved over time and have undergone significant changes in their morphology and genetics.
- Sago Palm is a cycad plant that is native to Japan and is widely cultivated for its starchy edible seeds.
- Sargasso is a type of seaweed that is found in the Sargasso Sea and is an important habitat for many marine organisms.

In conclusion, the only plant species that has remained relatively unchanged over the last several million years is the Maiden Hair Tree or Ginkgo biloba.

The plant which has undergone less or no evolutionary change within geological time scale is known as Living fossil. Maiden hair tree is one such example. option A is correct.