Comparison of stability of cyclic imines I am looking for the reason for the lowest stability of cyclohexane-1,3,5-triimine (D).

I was not able to find any explanation using resonance or aromaticity.
All help is appreciated.
The cyclohexane-1,3,5-triimine tautomer is benzene-1,3,5-triamine, which is aromatic. This makes this trienamine form much more stable than any of the other possible tautomeric forms.