Non-negotiable certificates of deposit are:

Non-negotiable certificates of deposit (CDs) are designed to be held until maturity, which makes them illiquid compared to negotiable CDs. The term "non-negotiable" implies that these instruments cannot be easily bought or sold in the secondary market, unlike their negotiable counterparts, which can be transferred between parties before maturity. This inherent characteristic of non-negotiable CDs, combined with often longer maturity periods, contributes to their illiquid nature, meaning that investors may not be able to access their funds until the CD matures without incurring penalties.

In contrast, options that suggest they are available on the secondary market or volatile derivatives mischaracterize non-negotiable CDs, as they do not possess those traits. Furthermore, while there is backing by banks or financial institutions, they are not guaranteed by the U.S. government, as stated in one of the other choices. Hence, the classification of non-negotiable certificates of deposit as illiquid aligns with their nature and the expectations for investors holding such instruments.