Final answer:
Demerol is safe to administer during active labor (option 'a'), which is before the transition phase and the second stage. The hormone primarily responsible for contractions during labor is oxytocin.
Step-by-step explanation:
When caring for a client in labor, it's important to know the correct timing for the administration of pain medication. The safe time to administer Demerol, a pain relief medication, is during active labor, which corresponds to your option 'a'. Active labor is the time when the cervix dilates from about 3 to 7 centimeters, and it is before the transition phase, which is the final part of active labor, where the cervix dilates from 7 to 10 centimeters. Administering Demerol during the second stage of labor or during the transition phase (option 'b' and 'c', respectively) can have adverse effects on the mother and the neonate. The second stage begins with complete cervical dilation and ends with the delivery of the baby. Demerol should also not be administered before the onset of contractions (option 'd'), as it would not be effective or appropriate at this time. It is during this first stage of labor, as the cervix is thinning and dilating, that the medication can be given safely to manage pain.
As for the hormone primarily responsible for the contractions during labor, the answer is 'oxytocin' (option 'a'). Before the baby and the placenta are expelled, the cervix thins and dilates, which is a crucial process for childbirth, and contractions of the uterus occur to push the baby out during the second stage.