Final answer:
Australia is estimated to have between 600,000 and 700,000 species of plants and animals, with vascular plant species estimated to be around 20,000.
Step-by-step explanation:
The number of vascular plant flora in Australia is quite significant given the country's unique biodiversity and the range of habitats present. While vascular plants are a large group consisting of various species, specifically within Australia, there has been a substantial study conducted to quantify the number of species. According to A.D. Chapman's report for the Australian Biological Resources Study, titled "Number of Living Species in Australia and the World," biologists estimate that Australia has a total of between 600,000 and 700,000 species of plants and animals, of which approximately three-quarters are endemic species found solely in Australia. From this data, the support for the number of Australia's vascular plants would more accurately align with option B) 20,000 species.