Instruments providing for the payment of money executed by or on behalf of any state of the United States or the District of Columbia or any governmental entity organized by or under the laws of any state of the United States or the District of Columbia, to carry out a public governmental or proprietary function, acting through its corporate authorities, or that any governmental entity has assumed or agreed to pay and that, at the time of investment, have been given one of the top 4 rating grades by a nationally recognized rating service.