The district may invest any surplus money in its treasury, including money in any sinking fund, in any of the following:
(a) Its own bonds.
(b) Treasury notes, certificates of indebtedness, bills, bonds of the United States, or any other evidence of indebtedness secured by the full faith and credit of the United States.
(c) Obligations issued pursuant to the Federal Home Loan Bank Act (12 U.S.C. Sec. 1421) or the National Housing Act (12 U.S.C. Sec. 1701).
(d) Treasury notes or bonds of this state, or of any public corporation, municipal corporation, public district, or political subdivision within this state which are legal as security for the deposit of public funds.