The correct answer is option 'D' - Methanogens.

Explanation:
Methanogens are primitive prokaryotes that are responsible for the production of biogas from the dung of ruminant animals. Biogas is a mixture of gases, primarily methane (CH4) and carbon dioxide (CO2), that is produced through a process called anaerobic digestion.

Anaerobic digestion occurs in the digestive system of ruminant animals, such as cows, sheep, and goats. These animals have a specialized digestive system that allows them to break down complex carbohydrates found in plant material, such as cellulose and hemicellulose, into simpler compounds that can be digested and utilized for energy.

Methanogens are a group of archaea that can thrive in anaerobic environments, such as the rumen of ruminant animals. They play a crucial role in the anaerobic digestion process by converting the byproducts of carbohydrate fermentation, such as volatile fatty acids and hydrogen, into methane gas.

The process of biogas production involves the following steps:

1. Fermentation of carbohydrates: In the rumen, bacteria and protozoa break down complex carbohydrates present in the dung of ruminant animals into simpler compounds, such as volatile fatty acids, hydrogen, and carbon dioxide.

2. Acetogenesis: Acetogenic bacteria convert the volatile fatty acids into acetic acid and other simple organic acids.

3. Methanogenesis: Methanogens utilize the acetic acid, hydrogen, and carbon dioxide produced in the previous steps to produce methane gas. Methanogens are unique in their ability to convert carbon dioxide and hydrogen into methane through a process called methanogenesis.

4. Collection and utilization of biogas: The methane gas produced by methanogens is collected and used as a source of renewable energy. Biogas can be used for heating, cooking, and electricity generation.

In conclusion, methanogens are the primitive prokaryotes responsible for the production of biogas from the dung of ruminant animals. They play a vital role in the anaerobic digestion process by converting the byproducts of carbohydrate fermentation into methane gas.