Final answer:
The largest ecosystem in the world is the ocean, which is home to two-thirds of all species. Islands are particularly vulnerable to exotic species. Option C is correct.
Step-by-step explanation:
The largest ecosystem in the world is the ocean where two-thirds of all species live. Therefore, the correct answer is c) Ocean. Oceans cover about 70% of the Earth's surface and are home to a vast array of life forms, from microscopic plankton to the massive blue whale.
The diversity of habitats within the ocean, such as coral reefs, deep-sea vents, and open water, provides for a immense diversity of species. Furthermore, exotic species are especially threatening to island ecosystems, because these ecosystems tend to be more isolated and have species that are often not adapted to competition or predation from outsiders.