(a) six percentum per annum,
(b) the rate prescribed by the superintendent of financial services pursuant to section fourteen-a of the banking law,
(c) the rates of mortgages or mortgage bonds insured by the federal housing administration or any other instrumentality of the federal government and
(d) such rate as may be approved by the supervising agency provided, however, that the applicable rate for purposes of paragraphs (b), (c) and (d), of this section one hundred eleven shall be the rate applicable or approved at the time the redevelopment company incurs the mortgage indebtedness. As used in this section the term "bond" includes a note heretofore or hereafter made.