Chloroform, trichloromethane, undergoes slow aerial oxidation in the presence of light to form the extremely toxic gas phosgene, also called carbonyl chloride, COCl2. To prevent this, chloroform is stored in dark coloured bottles filled to the brim to exclude air, and about one percent ethanol is added to convert any phosgene formed into harmless ethyl carbonate. Carbon tetrachloride is wrong because it is a separate, fully substituted compound and not the oxidation product of chloroform. Formic acid is incorrect since the oxidation does not stop at a carboxylic acid; the chlorine atoms remain and give an acyl chloride type product. Dichloromethane is wrong because oxidation adds oxygen and removes hydrogen rather than simply removing one chlorine. This matches the NCERT account of why chloroform is stored carefully. Sanity check: replacing the lone C-H of CHCl3 with a C=O and losing one Cl gives COCl2, consistent with phosgene as the oxidation product.