In the list of animals given below, hen is the odd one out.

human being, cow, dog, hen

The reason for this is

In the given list of animals, hen is the odd one because it is the only one which lays eggs, that is it shows oviparity. Humans, cow and dog are viviparous, i.e. they give birth to young ones.

All four organisms show internal fertilization, none of them fertilizes externally, thus these cannot be reasons for the hen being the odd one.