The correct option is D −H<−D<−T<−CH3
−CH3 group has the highest tendency to donate electrons, so it has the highest +I effect.
In case of isoptopes, heavier isotopes have more +I effect.
Comparing C−H and C−D bonds:
A shorter bond i.e. C−D bond has less volume to spread the electron density, resulting in a higher electron density throughout the bond Consequently, more is the electron density at carbon end of the bond and hence −D has more +I effect.
Similar arguments can be applied to tritium (T) Since, C−T bond is the shortest, it should be even more inductively electron donating towards carbon than deuterium.
So, the correct order is :
−H<−D<−T<−CH3