For a firm with publicly traded bonds outstanding, the pre-tax cost of debt:

is equivalent to the average current yield on all of the firm's outstanding bonds.

is equal to the coupon rate on the bonds issued by the firm in the past.

is based on the original coupon rate on the bonds issued by the firm last year.

is based on the current yield to maturity of the firm's outstanding bonds.

has to be otherwise estimated as it cannot be directly observed in the market.