(a)The monomer NO2exists in equilibrium with its dimer N2O4.On cooling the unpaired electrons of 2 NO2molecules combine together to form N-N bond in N2O4.

(b)ICl is more reactive than I2
This is because inter- halogen compounds are in general more reactive than halogens due to weaker inter- halogen X-X bond than X-X bond.
So, I2is more stable and less reactive than ICl.