**Methanogens and Their Classification**

Methanogens are a group of microorganisms that belong to the domain Archaea. They are prokaryotic organisms that are capable of producing methane as a metabolic byproduct. Methanogens are anaerobic organisms, meaning they thrive in environments devoid of oxygen. They are found in diverse habitats such as wetlands, marshes, ruminant digestive tracts, and even in the deep-sea hydrothermal vents.

**Classification of Methanogens**

Methanogens are classified based on their energy and carbon sources. Let's discuss the different options given and determine the correct classification for methanogens:

a) Photoautotrophs: Photoautotrophs are organisms that use light energy to convert carbon dioxide into organic compounds. They are typically found in environments where light is available, such as in the presence of sunlight or other forms of radiant energy. However, methanogens do not use light energy for their metabolic processes, so they cannot be classified as photoautotrophs.

b) Saprotrophs: Saprotrophs, also known as decomposers, obtain their energy and nutrients by breaking down dead organic matter. They play a crucial role in the recycling of nutrients in ecosystems. While methanogens do contribute to the decomposition process by breaking down organic matter, they do not solely rely on dead organic material as their energy source. Therefore, methanogens cannot be classified as saprotrophs.

c) Chemoautotrophs: Chemoautotrophs are organisms that use inorganic compounds as a source of energy to convert carbon dioxide into organic compounds. Methanogens fall under this category as they utilize a unique metabolic pathway to produce methane. They obtain energy by converting hydrogen gas (H2) or other small organic molecules into methane (CH4). This process is known as methanogenesis, and it occurs in the absence of oxygen. Methanogens are the only group of microorganisms known to produce methane as a metabolic end product, making them distinct chemoautotrophs.

d) None of these: The correct answer is not "none of these" because methanogens can indeed be classified as chemoautotrophs.

**Conclusion**

In conclusion, methanogens are classified as chemoautotrophs. They are unique microorganisms that use inorganic compounds as an energy source to convert carbon dioxide into methane. Their ability to thrive in anaerobic environments and produce methane has significant implications for various ecosystems and industrial processes.

Chemoautotrophs