Final answer:
Partial placenta previa occurs when the placenta partially covers the cervical opening, leading to potential complications like antepartum hemorrhage.
Step-by-step explanation:
In partial or incomplete placenta previa, the placenta is implanted in the lower uterine segment and partially covers the cervical os (cervical opening). This condition can lead to complications such as antepartum hemorrhage, which is profuse vaginal bleeding occurring after week 24 of pregnancy but before childbirth. The correct answer to the student's question is: a) Partial covering of the cervical os by the placenta.