$\mathrm{CHCl_3}$ (chloroform) on prolonged storage in the air may be partially oxidised to a HIGHLY POISONOUS gas. The product is

1. NCERT §6.6 (Polyhalogen compounds — chloroform): on exposure to AIR and LIGHT, $\mathrm{CHCl_3}$ is slowly oxidised. 2. The reaction: $2\,\mathrm{CHCl_3} + \mathrm{O_2} \to 2\,\mathrm{COCl_2} + 2\,\mathrm{HCl}$. 3. The product $\mathrm{COCl_2}$ is PHOSGENE — a highly TOXIC gas (used as a chemical weapon in WWI). 4. To prevent this, chloroform bottles are filled to the top (excluding air), stored in dark amber bottles, and a small amount of ETHANOL is added — ethanol scavenges any phosgene that forms. 5. Option A is the over-chlorinated form. Option C is a by-product of the same reaction (but not the toxic species named). Option D doesn't form here. _Source: NCERT Class 12 Chemistry Part 2, Ch 6, §6.6 (Chloroform — storage precautions), p. 16._