Correct Answer - Option 2 : coliform group of bacteria
Explanation:
Testing and counting of pathogens (disease-causing micro-organisms) in raw water can be done with great difficulty as tests involved are very costly.
Hence, in order to minimize the cost, instead of testing the raw water for pathogens it is tested for coliforms, which themselves are harmless micro-organisms but their presence or absence indicates the presence or absence of pathogens in raw water.
The Coliforms like E-Colli and B-Colli are the most common types which are used for this purpose.
Testing of Coliforms can be done by using methods such as Membrane filter technique, Most Probable Number (MPN) and B-Colli Index etc.
