Final answer:
In the case of a 39-year-old G1 in labor with full dilation and the fetus at +2 station, the correct intervention is to apply either a vacuum or forceps to assist with delivery.
Step-by-step explanation:
The most appropriate next step in the management of a 39-year-old G1 who is in labor at term, 10 cm dilated, with the fetus in left sacrum anterior at +2 station and an estimated fetal weight of 3200 g, is to apply a vacuum or forceps. Considering that the cervix is fully dilated and the fetus has descended into the pelvis, a Cesarean section would not be indicated unless there are signs of fetal distress or the labor fails to progress. Internal or external versions are not appropriate at this stage of labor, as the baby is already significantly engaged in the birth canal. Forceps or a vacuum assist can be used to facilitate the delivery of the fetus when the mother is fully dilated and the baby is in an adequate position but may require assistance to be born.