Final answer:
Prokaryotes, including both Bacteria and Archaea domains, are the most numerous organisms on Earth due to their ability to thrive in a wide variety of habitats, including extreme conditions.
Step-by-step explanation:
The organisms that are most numerous on Earth are prokaryotes. This includes the two domains of life: Bacteria and Archaea. Prokaryotes are single-celled organisms that lack a nucleus and other membrane-bound organelles. They were the first forms of life on the planet, appearing approximately 3.5 to 3.8 billion years ago. They are extremely abundant and can be found in a wide range of environments, including extreme conditions such as boiling springs and frozen environments. Their ubiquity and sheer numbers make prokaryotes the most numerous organisms on Earth.
Other options such as insects, plants, eukaryotes (which include animals, plants, fungi, and protists), and archaea (which are included in prokaryotes but are not as numerous as the whole prokaryotic group) do not match the sheer quantity of prokaryotic organisms.