Among vertebrates, the unique feature of lampreys and hagfish is the among vertebrates, the unique feature of lampreys and hagfish is the absence of jaws absence of a postanal tail absence of a backbone presence of a cartilaginous skeleton

The correct answer is absence of jaws.
Lampreys and hagfish are in the class Agnatha, they are jaw-less fish. The members of the agantha class are probably the earliest vertebrates. Scientists have found fossils of agnathan species from the late Cambrian period that occurred 500 million years ago.