Birbal Sahni worked on -
(a)Algae
(b)Bryophytes
(c)Fossil plants
(d)Angiosperms

Hint: Birbal Sahni was a paleobotanist and a major contributor to botanical science. His work was based on archaeology, geology, and botany of the Indian subcontinent. Hence, his discovery is something that is highly relevant to the field of archaeology and paleobotany.

Complete answer:
 - Birbal Sahni was a paleobotanist, the field of studying fossilized plants of the Indian subcontinent. He is known for the revision of Indian Gondwana plants.
- He founded a new group of fossil gymnosperms, the Pentoxylales. He also found a new class of fossil plants - Homoxylon rajmalzalense - In the Raj Mahal hills of Bihar.
- He was among the first botanists to suggest a separate order, the Taxales, within the conifers to contain the genera Taxus, Torreya, and Cephalotaxus.

Additional Information: - He is known for contributing to the knowledge of Nephrolepis, Niphobolus, Taxus, Psilotum, Tmesipteris, and Acmopyle species.
- He was also appointed as a fellow of the Royal Society, London.
- He founded the Birbal Sahni Institute of Palaeobotany at Lucknow in 1946.
- He also predicted the relationship of the residents of the Harappa civilization to those regions where conifer trees were grown.
- Paleobotany also includes the study of prehistoric marine photoautotrophs, such as photosynthetic algae, seaweeds, or kelp, apart from terrestrial plant fossils.

So, the correct answer is ‘Fossil plants’.

Note: - The Birbal Sahni Gold Medal for students of botany was instituted in his memory and awards students who excel in botany.
- Examples of plants that are living fossils are Ginkgo biloba, Sciadopitys verticillata, horsetails, dawn redwood, tree ferns, and ferns.