What feature allows a bondholder to retire a bond before its maturity date?

The correct answer is the call feature. This feature is specifically designed to give the issuer of the bond the right to redeem the bond prior to its maturity date. Typically, an issuer might choose to call the bond when interest rates have declined, as it allows them to refinance their debt at a lower cost, thereby saving money on interest payments.

In contrast, a put option is a feature that allows bondholders to sell their bonds back to the issuer at a predetermined price on specified dates before maturity. The conversion option permits bondholders to convert their bonds into a specified number of shares of the issuer's stock, and the redemption feature generally refers to the overall process of paying back the bond's face value upon maturity. Thus, the call feature uniquely empowers the issuer to retire the bond early, making it the correct answer.