The correct order of decreasing length of the bond as indicated by the arrow in the following structures is

The length of carbon-carbon single bond is always greater than that of the carbon-carbon double bond. In III, positive charge is not in conjugation of double bond, so the bond does not acquire a double bond character. However, chances of acquiring double bond character fof the indicated bond) is much more in I than that in case of II. Thus, the correct order of decreasing bond length is