In option (d) sigma-resonance is operating, then why is option (b) the most stable carbocation?
The most stable carbocation is

Dear Student,

The stability of carbocation ranking is tertiary > secondary > primary and its stability are increased based on the adjacent molecules in the given molecule aromatic with -N element. The stability of carbocation is enhanced by N⁺ molecule in the adjacent atoms hence Option (B) is more stable carbocation molecule.

Regards.