hey these are my answers

 

a> chlorobenzene

 i found out its reason of answer as it seems in chloro benzene event though -I effect dominates more that +M effect but still it is preferred for sulphanation alone it seems as a dipolar attraction is created between the CL and S which stabilises the compund it seems i read in one site!!!!

 

b>ofcourse it must be m xylene as more the number of +R groups more the strength for nitration but mesitylene is not the answer due to steric reasons!!!!

 

c> the answer here too is obvious its benzene triozonide as when once aromaticity is lost why shouldnt the compund becomes highly unstable and thus rapidly decomposes into bezene triozonide cleaving all the double bonds you never get the other two!!!!

any contradictions!!!????