Assertion :Vigorous contraction of the uterus at the end of pregnancy causes expulsion. Reason: The stimulatory reflex between the uterine contraction and oxytocin secretion results in weakening contractions.

The correct option is C Assertion is correct but Reason is incorrect
Vigorous contraction of the uterus at the end of pregnancy causes parturition. Parturition is induced by a complex neuroendocrine mechanism. The signals for parturition originate from the fully developed fetus and the placenta which induce mild uterine contractions called fetal ejection reflex. This triggers a release of oxytocin from the maternal pituitary. Oxytocin acts on the uterine muscle and causes stronger uterine contractions, which in turn stimulates further secretion of oxytocin. The stimulatory reflex between the uterine contraction and oxytocin secretion continues resulting in stronger and stronger contractions.

So, the correct answer is 'Assertion is correct but Reason is incorrect'.