Order of increasing +I effect of the following is: I. −O⊖  II. CH33C−  III. CH32CH−  IV. C2H5−

Atoms or groups which release electrons towards a carbon atom are said to have a +I effect. Methyl group release electrons towards carbon. i.e., it has +I effect. +I effect increases as the number of methyl groups increases.has lone pair of electrons, with negative charge, so it has maximum +I effect.