Biodiversity is best described as c) all species present in an ecosystem.
Explanation: Biodiversity refers to the variety and variability of life on Earth, encompassing the range of species, genetic diversity, and the ecosystems they form. While microscopic organisms and physical differences between species are important aspects of living organisms, the term ‘biodiversity’ primarily emphasizes the total number of species and their interactions within a particular ecosystem. Therefore, option c captures the essence of biodiversity more accurately than the other options.