The answer is A. Decomposers.
Decomposers are organisms that break down dead organisms and organic wastes into simpler substances that can be reused by other organisms. They are essential for the recycling of nutrients in an ecosystem.
Producers are organisms that can make their own food from inorganic substances. They are the first link in the food chain and are essential for the production of organic matter in an ecosystem.
Primary consumers are organisms that eat producers. They are the second link in the food chain and are important for the transfer of energy from producers to other organisms.
Secondary consumers are organisms that eat primary consumers. They are the third link in the food chain and are important for the transfer of energy from primary consumers to other organisms.
Decomposers are the only organisms that can break down dead organisms and organic wastes into simpler substances that can be reused by other organisms. They are essential for the recycling of nutrients in an ecosystem.