−NO2 group is an electron withdrawing group and exhibit −I-effect. This effect increases with decrease in distance of positive charge present on C-atom and, hence lesser is the stability of carbocation.
In option (a), the positive charge is at maximum distance to NO2 group, so −I effect due to NO2 group will be minimum and stability will be maximum.
In option (b) and (d), the positive charges is at minimum distance to NO2 group, hence the stability will be minimum.
Also, in option (c), the distance of positive charge to NO2 group is more than ortho but less than para, so it will be less stable as compared to option (a).
This resonating structure is less stable than option (a)
