Final answer:
Emission of carbon dioxide from sustained volcanic eruptions during the Cretaceous period is an example of a natural process that led to long-term global warming due to the lengthy atmospheric residence time of greenhouse gases.
Step-by-step explanation:
An example of a natural process that created long-term global warming is c) emission of carbon dioxide from sustained volcanic eruptions during the Cretaceous period.
Unlike short-lived volcanic events which typically cool the climate by releasing particles that block sunlight and cause haze-effect cooling, long-term emissions of greenhouse gases like carbon dioxide can lead to a warming of the planet.
This is because greenhouse gases, once in the atmosphere, can remain there for extended periods, trapping heat and resulting in an overall increase in global temperatures.