a. the numbering should be such that all the substituents get the least numbering possible.

b. out of the substituents –Cl, -NO₂ and –CH₃ , nitro group (-NO₂ ) gets the priority, meaning nitro benzene is the parent compound and rest are substituents.

c. Least possible numbering is possible only when the carbon with –Cl is designated position one, then the carbon with -NO₂ is given two and the carbon with –CH₃ is given four.