In which one of the following, methano- gens, i.e., methane producing

In which one of the following, methano- gens, i.e., methane producing bacteria are not present ?

Rumen of cattle
Aerobic environment
Anaerobic sludge
Excreta of cattle
This question was previously asked in
UPSC NDA-2 – 2016
Methanogens are a group of anaerobic archaea that produce methane as a metabolic byproduct in anoxic conditions. They are strictly anaerobic, meaning oxygen is toxic to them.
Methanogens thrive in environments where oxygen is absent and organic matter is undergoing decomposition. Examples include the digestive tracts of ruminants (like cattle rumen), anaerobic sludge digesters in wastewater treatment, and sediments in swamps and marshes.
The rumen of cattle is a highly anaerobic environment where methanogens play a crucial role in the digestion of cellulose. Anaerobic sludge in sewage treatment plants is another common habitat. Cattle excreta, being partially digested material from the rumen and exposed to anaerobic conditions during storage, also contains active methanogens. An aerobic environment, however, is unsuitable for methanogens due to the presence of oxygen.