The trustees shall have the custody of all trust funds held by their town. Any person who directly or indirectly receives any such trust funds for deposit or for investment in securities of any kind shall, prior to acceptance of such funds, make available at the time of such deposit or investment an option to have such funds secured by collateral having value at least equal to the amount of such funds. Such collateral shall be segregated for the exclusive benefit of the town depositing or investing such funds. Only securities defined by the bank commissioner, as provided by rules adopted pursuant to N.H. Rev. Stat. § 383-B:3-301(e), shall be eligible to be pledged as collateral. The funds shall be invested only in deposits in federally insured banks authorized to accept deposits under N.H. Rev. Stat. § 6:8, I and I-a, or in deposits in any credit union in this state, or in state, county, town, city, school district, water and sewer district bonds and the notes of towns or cities in this state; and such stocks and bonds as are legal for investment by any bank chartered by this state to engage in a banking business; and in participation units in the public deposit investment pool established pursuant to N.H. Rev. Stat. § 6:45; or in obligations with principal and interest fully guaranteed by the United States government. The obligations may be held directly or in the form of securities of or other interests in any open-end or closed-end management-type investment company or investment trust registered under 15 U.S.C. § 80a-1 et seq., if the portfolio of the investment company or investment trust is limited to such obligations and repurchase agreements fully collateralized by such obligations. Deposits in a federally insured bank or credit union shall be made in the name of the town which holds the same as a trust, and it shall appear upon the books thereof as a trust fund. Shares of mutual funds are also permitted if they are registered with the Securities and Exchange Commission, qualified for sale in the state of New Hampshire in accordance with the New Hampshire uniform securities act of the New Hampshire secretary of state’s office, and which have in their prospectus a stated investment policy which is consistent with the investment policy adopted by the trustees of trust funds in accordance with this chapter, and when so invested, the trustees shall not be liable for the loss thereof. The trustees may retain investments as received from donors, until the maturity thereof. The trustees shall formally adopt an investment policy for all investments made by them or by their agents for any trust funds in their custody in conformance with the provisions of applicable statutes. Such investment policy shall be reviewed and confirmed at least annually. A copy of the investment policy shall be filed with the attorney general.