The correct answer is A. 1.
The relative stability of a sewage sample is a measure of how well the sample can resist changes in dissolved oxygen levels. A sample with a relative stability of 1 is perfectly stable, meaning that the dissolved oxygen level will not change even if the sample is exposed to a large amount of organic matter. A sample with a relative stability of 0 is completely unstable, meaning that the dissolved oxygen level will drop to zero very quickly if the sample is exposed to a large amount of organic matter.
The total oxygen required to satisfy BOD is the amount of oxygen that is needed to break down all of the organic matter in the sample. If the dissolved oxygen level in the sample is the same as the total oxygen required to satisfy BOD, then the sample is perfectly stable and will not experience any changes in dissolved oxygen levels.
Option B is incorrect because 100 is not a valid value for relative stability. The relative stability of a sample must be between 0 and 1.
Option C is incorrect because infinite is not a valid value for relative stability. The relative stability of a sample must be between 0 and 1.
Option D is incorrect because zero is the value for relative stability when the sample is completely unstable.