In Choose odd pair of words type of questions, certain pairs of words are given out of which the words in all the pairs except one, bear a certain common relationship. The candidate is required to decipher this relationship and chose the pair in which the words are differently related, as the answer.
(Cow : Calf), (Dog : Bitch), (Lion : Cub), (Tortoise : Turtle), (Insect : Larva)
Clearly, the answer is (b).
In all other pairs, second is the young one of the first while in (b), second is the female of the first.