Answer:
The correct answer would be "They release the carbon that remains in the bodies of dead organisms".
Decomposers are heterotrophic organisms which get their nutrition by breaking down remains of dead plants and animals. The process of breaking down the dead remains of plants and animals is termed as decomposition.
By doing so, they play an important role in an ecosystem. They help in recycling of the nutrients such as carbon and nitrogen in an ecosystem. They return these nutrients to the soil from where plants can reuse the same.
In addition, they help in removing or cleaning the dead matter and waste material from the ecosystem.
