Correct Answer - Option 2 : 0.05 to 0.5 ppm
(i) Doses of chlorine should be sufficient so as to leave a residue of 0.2 mg / l or 0.2 ppm after 10 minutes of the contact period.
(ii) The residue chlorine is tested by the Diethyl-paraphenylene diamine (DPD) test.
So, as 0.2 ppm is in the range of 0..05 – 0.5 ppm
∴ option (b) is the answer.