It should be when A. late in the third trimester, specifically months 7 and 8. It can't be B or D because the baby is breathing and getting oxygen through the placenta. It's not C because the lungs will de fully developed by the time the baby is out. In the 3rd trimester "the lungs are still developing still shortly before birth, but rhythmic breathing movements begin before lungs are fully developed." Its in my notes. I hope this helps!
