The correct answer for the blank is-
The class mammalia contains the only animals that have mammary glands.
Mammalia is considered as class of higher vertebrates (such as , marsupials, placentals, or monotremes) that provide nourishment to their offspring through mammary glands, which secrete milk.
They are warm blooded organisms and their skin is usually covered with more or less hairs.
Example- Human beings.
Have mammary glands - GradPoint answer (you can do this)
Oxytocin causes contractions during pregnancy, it also is the reason why mothers feel "motherly" towards their children after childbirth as there are high levels on oxytocin in the mother's blood.
Check the pic