Final answer:
A mass extinction is defined as the widespread and rapid extinction of many species, and it represents a severe loss of biodiversity often linked to catastrophic events or significant environmental changes.
Step-by-step explanation:
A mass extinction is defined as (d) The widespread and rapid extinction of many species. It is when a significant percentage of species on Earth disappears in a geologically short period of time. This can be due to various catastrophic events or changes in the environment, and can have a considerable impact on the planet's biodiversity.
Mass extinctions have occurred throughout Earth's history, with the fossil record showing five major events prior to human influence. However, the current rate of species loss is alarming and is often compared to these historical mass extinctions—this time mainly due to human activities such as habitat destruction, introduction of exotic species, and over-harvesting. The resulting decline in biodiversity poses a significant threat to the resilience of ecosystems.