The test used for determining the biological quality of drinking water is the coliform test.
Option 1: Acidity test - This test is used to measure the acidity level of water. While it can indicate the overall chemical quality of water, it does not specifically assess its biological quality.
Option 3: Iodine test - This test is used to determine the presence of iodine in water. It is primarily used to check if water has been properly disinfected with iodine, but it does not evaluate the biological quality of water.
Option 4: Vidal test - There is no test called the Vidal test specifically used for determining the biological quality of drinking water. Therefore, this option is incorrect.
The coliform test, option 2, is widely used to assess the biological quality of water. Coliform bacteria are commonly found in the environment and their presence in drinking water can indicate possible contamination by other pathogens. This test helps to identify any potential health risks associated with the water and is an important step in ensuring safe drinking water for consumption.
Therefore, the correct answer is option 2 - Coliform test.