An oxidising agent is needed in the iodination of methane but not in the chlorination or bromination . Assign reason-

In the iodination of methane, H-I is also formed as the product along with iodomethane. Since it is a strong reducing agent, it reduces iodomethane back to methane and makes the reaction reversible. In order to destroy HI, oxidising agents like HIO₃(or HNO₃) is needed. But HCl and HBr formed in the chlorination and bromination reaction of methane are not in a position to react with the monosubstituted product, Since they are comparatively weak reducing agents. Therefore, no oxidising agents are needed for these reactions. 

CH₄+ I₂ ------> CH₃I+ HI 

(Methane) 

5HI + HIO₃ ----------> 3H₂O + 3I₂