Each loan with respect to which a guarantee is made or interest subsidies are paid under this subpart shall be secured in a manner which the Secretary finds reasonably sufficient to insure repayment. The security may be one or a combination of the following:

(a) A first mortgage on the facility and site thereof.

(b) Negotiable stocks or bonds of a quality and value acceptable to the Secretary.

(c) A pledge of unrestricted and unencumbered income from an endowment or other trust fund acceptable to the Secretary.

(d) A pledge of a specified portion of annual general or special revenues of the applicant acceptable to the Secretary.

(e) Such other security as the Secretary may find acceptable in specific instances.