During the process of parturition, the pituitary hormone .............. signals the uterus to contract.

The correct option is A Oxytocin
Oxytocin is a mammalian hormone produced by the posterior pituitary gland. It is released in small amounts after distension of the cervix and uterus during labor, facilitating birth. It also plays an important role in milk production in lactating mothers.
Thus, the correct answer is option (A), 'Oxytocin'.