Which of the following is NOT a category of bonds?

In financial terminology, bonds can be categorized based on various characteristics, and the options provided include some commonly recognized classifications. The term "sustainable" is not typically used as a category of bonds in conventional finance literature.

Irredeemable bonds, also known as perpetual bonds, are a type of bond that does not have a maturity date and pays interest indefinitely. Redeemable bonds, on the other hand, are those that can be repaid before their maturity date, typically at the issuer's discretion. Vanilla bonds refer to traditional bonds with standard features, such as fixed interest payments and a specified maturity date.

Since "sustainable" does not fit into these common categories and lacks a standardized definition in the context of bond classifications, it is identified as the correct choice for the question regarding what is NOT a category of bonds. This reinforces the importance of familiarizing oneself with terminology in the field of finance, particularly when discussing investment products like bonds.