A group of researchers procured some fish-like animals and upon investigation the following characters were observed:

A. Endoskeleton was made of cartilage.
B. Ectoparasitic; as they were found attached on fish skin with their circular sucking mouth.
C. Paired fins and scales are absent, but 7 pairs of gill slits were present.
Which of the following species of animals did they consider to fit best with these characters?

First, list the observed characters:

A. The endoskeleton is made of cartilage.
B. The animal is ectoparasitic, found attached to fish skin with a circular sucking mouth.
C. Paired fins and scales are absent, but there are 7 pairs of gill slits.

Next, compare each character with the four candidate groups.

1. Petromyzon sp. (lampreys)
- Lampreys possess a cartilaginous endoskeleton, satisfying character A.
- They are well‑known ectoparasites that use a round, suction‑cup mouth to attach to the bodies of other fish, matching character B.
- Lampreys lack paired fins and true scales; instead they have a smooth, eel‑like body. They have multiple gill openings—typically seven pairs—fulfilling character C.

2. Branchiostoma sp. (amphioxus)
- Amphi­oxus have a notochord rather than a true cartilage skeleton, so character A is not met.
- They are free‑living filter feeders, not ectoparasites, violating character B.
- They possess a series of gill slits but also retain a dorsal fin‑like structure, not a perfect fit for character C.

3. Scoliodon sp. (a shark)
- Sharks have a skeletal framework of cartilage, satisfying A.
- Sharks are predators, not ectoparasites, contradicting B.
- Sharks have paired pectoral and pelvic fins and are covered with placoid scales, contrary to C.

4. Exocoetus sp. (flying fish)
- Flying fish have bony skeletons, not a purely cartilaginous one, failing A.
- They are free‑swimming surface dwellers, not ectoparasites, violating B.
- They possess paired pectoral and pelvic fins and typical fish scales, opposing C.

Only Petromyzon sp. meets **all three** observed characters. Therefore, the species that best fits the described characters is Petromyzon sp.