Humans have accelerated the pace of ______________ extinction by three orders of magnitude in the last few hundred years.

The correct answer is: Species

The IUCN Red List has estimated the number of extinctions over the last five centuries. It estimates that 900 species have gone extinct since 1500. Our estimates for the better-studied taxonomic groups are likely to be more accurate. This includes 85 mammal; 159 bird; 35 amphibian; and 80 fish species.
Over the past 50 years, global biodiversity loss has primarily been driven by activities like the clearing of forests for farmland, the expansion of roads and cities, logging, hunting, overfishing, water pollution and the transport of invasive species around the globe.