Select the pair of words from the given options that shares a similar…

Identify the relationship in the given pair, Shark : Fish.
A shark is biologically classified as a type of fish (specifically, a cartilaginous fish). So the relationship is "member : its biological class".
Now test each option against this same "member : its biological class" pattern:
Bat : Bird → A bat is a mammal, not a bird. (Wrong class)
Dolphin : Fish → A dolphin is a mammal, not a fish. (Wrong class)
Whale : Mammal → A whale is biologically classified as a mammal. (Correct class)
Iguana : Amphibian → An iguana is a reptile, not an amphibian. (Wrong class)
Hence Whale : Mammal is the only pair that mirrors the logic of Shark : Fish.