Step 1: What a connecting link is.
A connecting link is an organism that shows features of two different groups at the same time, so it acts like a bridge between them in evolution.
Step 2: Check Seymouria.
Seymouria has features of both amphibians and reptiles, so it is a true connecting link between Amphibia and Reptilia. This pair is correct.
Step 3: Check Eusthenopteron.
Eusthenopteron is a lobe-finned fish that shows features pointing towards the first amphibians, so it links fishes and amphibians. This pair is correct.
Step 4: Why the other items fail.
Peripatus connects Annelida with Arthropoda (and relates to Onychophora), not Mollusca, so that pairing is wrong. Cynognathus is a mammal-like reptile and is not a clean bridge between reptiles and birds, so that pairing is wrong too.
Step 5: Keep the valid pairs.
That leaves the two correct connecting links as B and D.
Step 6: Conclusion.
Therefore the correct answer is "B & D". \[ \boxed{\text{B and D}} \]