Answer:
D. association with bacterial species.
Explanation:
Methanogens -
These are the organisms , that falls under the category of the prokaryotes , in the domain archaea .
Methanogens are a type of microorganism , which is capable to produce methane as one of the by product during a hypoxic condition .
These organisms are found in the wetland , inside the digestive system of the humans and animals .
They have an important space for the treatment of the anaerobic wastewater .
Hence , from the question ,
The correct option is d.
