The term "sixth mass extinction" is used in the context of discussions about the current rapid decline in biodiversity and the potential for it to be as significant as the five major extinction events in Earth's history. Mass extinction events: Throughout Earth's history, there have been five major extinction events where a large percentage of species (estimated at 75% or more) disappeared in a geologically short period. Examples include the Cretaceous-Paleogene extinction event that wiped out non-avian dinosaurs. Sixth mass extinction: Scientists are concerned that human activities like habitat destruction, climate change, pollution, and overexploitation of resources are causing a mass extinction event that could be as severe as the previous ones. Thus, the correct answer is 'D'.