Explanation:

The correct answer is option 'B': most of the bacteria in sewage are saprophytic.

Saprophytic bacteria:
- Saprophytic bacteria are organisms that obtain their nutrients by decomposing organic matter.
- These bacteria play a crucial role in the breakdown of sewage and other organic waste materials.
- They feed on dead organic matter and convert it into simpler compounds through the process of decomposition.
- Examples of saprophytic bacteria include species of Bacillus, Pseudomonas, and Clostridium.

Parasitic bacteria:
- Parasitic bacteria are organisms that live and feed on the host organism, causing harm or disease.
- They are not commonly found in sewage because sewage primarily consists of waste products and does not provide a suitable environment for parasitic bacteria.
- Examples of parasitic bacteria include species of Mycobacterium (which causes tuberculosis) and Salmonella (which causes food poisoning).

Anerobic bacteria:
- Anaerobic bacteria are organisms that can survive and grow without the presence of oxygen.
- While some anaerobic bacteria may be present in sewage, they are not the dominant type of bacteria in this environment.
- Anaerobic bacteria are more commonly found in environments such as deep sediments, swamps, and the digestive tracts of animals.

None of these:
- This option is not correct because bacteria in sewage are primarily saprophytic, as explained above.

Therefore, the correct answer is option 'B': most of the bacteria in sewage are saprophytic. These bacteria play a crucial role in the decomposition of organic waste materials and are essential for the treatment of sewage.