What is the correlation between the amount of available niches an ecosystem has and the amount of biodiversity it has?

The answer is; the more the niches the higher the biodiversity

Niche is the specificity of a species towards particular environmental conditions or resource. The adoption of a species towards a particular niche is mostly due to factors such as competition. Therefore the more the niches in the particular ecosystem the more the species occupying them.

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.

instead of copying and pasting things off of tests or work done on computer which im sure you did, try and read over the question carefully