Final answer:
Jaws arose earliest in the now-extinct class Placodermi, which led to the development of class Chondrichthyes. Placoderms were the first jawed vertebrates, marking a significant evolutionary advance from jawless fishes.
Step-by-step explanation:
The earliest class where jaws occurred is E) Placodermi. Placoderms are an extinct group of armored fishes and are thought to be among the first jawed vertebrates. As per evolutionary studies, jaws in vertebrates first appeared in gnathostomes, or "jaw-mouths," and placoderms were early gnathostomes. The class Chondrichthyes, which describes cartilaginous fishes like sharks, rays, and skates, descended from placoderms.
Unlike jawless fishes, such as the members of the superclass Agnatha including hagfish and lampreys, placoderms had true jaws which allowed them to be active predators. The evolution of jaws was a significant development in early vertebrate evolution as it enabled these creatures to grasp and tear their food, expanding their food options from the scavenging and suspension feeding seen in jawless fishes.