Nucleophilic substitution reactions generally expressed as `Nu^(-) +R-L rarr R-Nu +L^(-)` Where `Nu^(-) rarr` Nucleophile , `R-L rarr` substrate, `L rarr` leaving group The best leaving groups are those that become the most stable ions after they depart. since most leaving group leave as a negatibe ion, the best leaving groups are those ions that stabilize a negative charge most effectively. A good leaving group should be (a) electron-withdrawing to polarize the carbon (b) stable once it has left (not a strong base) (c) polaristable to maintain partial bonding with the carbon in the transition state (both `S_(N)1` and `S_(N)2)`. This bonding helps to stabilise the transition state and reduces the activation energy. (I) `CI^(-)` (II) `CH_(3)O^(-)` (III) `CH_(3)S^(-)` (IV) `I^(-)` The correct order of increasing leaving group capability of above anionsA. `III lt IV lt II lt I`B. `II lt III lt I lt IV`C. `II lt IV lt III lt I`D. `I lt III lt II lt IV`