Which diversity is found within a certain region or ecosystem, as measured by the number of species present (species richness)?

The correct answer is: Alpha diversity

Alpha diversity is the diversity within a specific region or environment and is typically reflected in the abundance of species inside that ecosystem.
Alpha diversity is a term used to describe the "within-sample" diversity. It's a measure of how diverse a single sample is, usually taking into account the number of different species observed. Alpha diversity metrics are also often weighted by the abundances at which the individual microbes are observed.