The best definition of the biosphere is b) all of the ecosystems on the planet.
The biosphere refers to the global sum of all living organisms and their relationships with the environments in which they reside. It encompasses not only the various ecosystems, such as forests, oceans, and deserts, but also how these ecosystems interact with one another and the wider physical environment. While options a, c, and d focus on smaller components or subsets of life, the biosphere captures the entirety of life on Earth and all its ecosystems functioning together.