The answer is serial bonds.
Callable bonds are bonds that can be redeemed by the issuer before their maturity date at a specified call price. This feature allows the issuer to repay the bond early, typically when interest rates have fallen, enabling them to refinance at a lower cost.
Convertible bonds are bonds that can be converted into a predetermined number of the issuer's equity shares. This feature provides the bondholder with the option to convert the bond into stock, usually at a time when the stock price has appreciated.
Serial bonds are bonds that mature in installments over a period of time. This means that portions of the total bond issue mature at different dates, spreading out the repayment schedule for the issuer.
Term bonds are bonds that mature on a single date. The entire principal amount is repaid at the end of the term, rather than in installments.
In this context, the correct answer is serial bonds, as they are specifically designed to mature in installments over a period of time.