The correct answer is (c). Green plants are an essential part of an ecosystem. They are the primary producers, meaning that they are the organisms that convert sunlight into energy through photosynthesis. This energy is then used by other organisms in the ecosystem, such as animals and bacteria. Green plants also play a role in regulating the climate by absorbing carbon dioxide from the atmosphere and releasing oxygen.
(a) Mountains are not the most stable ecosystems. They are constantly changing due to erosion and other natural processes.
(b) Bacteria are an example of an biotic component, which is a non-living part of an ecosystem. Other examples of abiotic components include water, soil, and air.
(d) Rainfall is not a correctly matched pair. Rainfall is a natural process that can affect ecosystems, but it is not an essential part of an ecosystem.