chapter name- gerneral organic chemistry …..kindly refer to the attachment

Dear Student,
The carbocation’s stablity depends on the stabilization of the positive charge.
The answer is (D)
It is because in ‘b’ carbocation the positive charge is stabilized by the lone pair conjugation of the vicinity Oxygen making it the most stable
The carbocation ‘a’ is second most stable because it is a secondary carbocation. It is stabilized by the inductive effect of the two methyl groups.
The carbocation ‘c’ is least stable because CO group is electron withdrawing and it destabilizes the positve charge.