In the list of animals given below, hen is the odd one out.Human being, cow, dog, hen.
The reason for this is
(a) it undergoes internal fertilisation
(b) it is oviparous
(c) it is viviparous
(d) it undergoes external fertilisation
(b) Hen is odd one out because it is oviparous, i.e. it lays egg from which young ones are hatched later on.
The rest three (human being, cow and dog) are viviparous, i.e. give birth to young ones after they are fully developed.