Explanation:

Let’s break it down:

- Cyclostomata:

- These are jawless vertebrates like lampreys and hagfish.

- They have a round, sucking mouth, no jaws, no paired fins or limbs.

- The skeleton is entirely cartilage, with a notochord that sticks around for life.

- They’re primitive, kind of eel-shaped, basically living fossils.

- Aschelminthes:

- This group is made up of roundworms.

- No backbone, they’re non-chordates.

- Definitely don’t fit the vertebrate, jawless fish description.

- Hemichordata:

- These are worm-like marine animals.

- Not true vertebrates; more related to echinoderms.

- They don’t have sucking mouths or real skeletons.

- Arthropoda:

- Think insects, crabs, spiders—creatures with exoskeletons and jointed legs.

- Zero notochord, zero vertebrate characteristics.

So when you’re talking about primitive vertebrates with a sucking jawless mouth and no limbs, Cyclostomata is the group. That’s the only answer that fits.