The order of decreasing stability of the carbanion at para position of benzyl anionI)CN      II)Cl      III)Vinyl      IV)Br

In the concept of stability of carbanion -I group plays crucial role; however, the extent of the effect is also important. Here the CN group as well as vinyl group can participate in the concept of resonance. But in the CN group the nitrogen atom can hold the negative charge more efficiently during the process of resonance, but in vinyl carbon can hold to less extent.