This is chiefly produced by the activity of an anaerobic bacteria on sewage

The correct option is Marsh gas.

Here is a step-by-step educational explanation of why this is correct:

1. Understanding sewage and anaerobic digestion:
Sewage contains a high concentration of organic matter. When sewage is treated or stored in the absence of oxygen (which is what "anaerobic" means), anaerobic microorganisms (specifically anaerobic bacteria) break down this complex organic material through a process called anaerobic digestion or fermentation.

2. Role of Methanogenic Bacteria (Methanogens):
During the final stages of anaerobic digestion, a specialized group of anaerobic bacteria known as methanogens (e.g., Methanobacterium) metabolize simple organic acids, alcohols, and hydrogen/carbon dioxide. This biological activity produces methane gas.

3. Identification of "Marsh gas":
"Marsh gas" is the common name for methane (chemical formula: ${\mathrm{CH}}_4$). It received this name because it is naturally produced in marshes and wetlands by the same anaerobic decomposition of plant and organic matter underwater, where oxygen levels are extremely low. Therefore, the gas chiefly produced by the activity of anaerobic bacteria on sewage is marsh gas (methane).