The original stimulus seen in the positive feedback loop occurring during child birth:

a. the hypothalamus releasing oxygen

b. oxytocin stimulating increased smooth muscle contraction

c. the stretching of the cervi by the fetal head

B. Oxytocin stimulating increased smooth muscle contraction

Explanation:

Parturition is the process of delivery of the foetus. Vigorous contractions of the uterus at the end of pregnancy causes the delivery of the foetus. It is induced by a complex mechanism which is a neuroendocrine mechanism. It is the fully developed foetus and the placenta which induces or signals the parturition by initiating the mild uterine contractions called foetal ejection reflex. This triggers the release of oxytocin from maternal pituitary gland. Oxytocin then acts on the uterine myometrium (made of smooth muscles) and causes stronger uterine contractions, which in turn stimulates further secretion of oxytocin. This reflex continues resulting in stronger and stronger contractions finally leading to expulsion of the baby out of the uterus through the birth canal.