In the molecule C, positive charge is present on the electronegative oxygen atom and is not having complete octet. Hence, the structure C is the least stable resonance structure. In the structure A, the delocalisation is taken place away from the electron withdrawing group, which is not favourable delocalisation. In the structure D, more number of covalent bonds are there and having all atoms' octet.

Order should be : $\mathrm{C}<\mathrm{A}<\mathrm{B}<\mathrm{D}$