The ability of a bond issuer to service its debt refers to______.

A. the issuer's agreement to convert the bond into common stocks at the bondholder's request
B. the issuer's willingness to issue a new bond whenever an existing bond is either lost or destroyed
C. the issuer's readiness to fix the coupon rate whenever interest rates are falling
D. the capacity of the issuer to make timely interest payments and repay the bond's principal on the scheduled maturity date