**Core Concept**
Placenta previa is a condition where the placenta partially or completely covers the cervical os, leading to painless vaginal bleeding in pregnancy. It is most commonly associated with uterine scarring or abnormal uterine architecture, particularly from prior uterine surgery.

**Why the Correct Answer is Right**
A previous cesarean section creates a scar in the uterine wall, which can lead to the placenta attaching over or near the cervix during subsequent pregnancies. This is due to the altered uterine anatomy and the tendency for the placenta to implant in areas of previous surgical trauma. The risk increases with each prior uterine surgery, especially cesarean delivery.

**Why Each Wrong Option is Incorrect**
Option A: Primigravida (first pregnancy) is actually less likely to have placenta previa than multiparous women, as the uterus is more flexible and less scarred.
Option B: Singleton pregnancy is not a risk factor; in fact, multiple gestations are more commonly associated with placenta previa, but this is not a direct predisposing factor.
Option C: Diabetes mellitus may increase the risk of complications like preeclampsia or macrosomia, but it does not directly predispose to placenta previa.

**Clinical Pearl / High-Yield Fact**
Women with a history of cesarean delivery have a 10–15% risk of placenta previa in subsequent pregnancies, and this risk increases with each prior uterine surgery. Always assess prior uterine surgery history in any patient presenting with painless vaginal bleeding in pregnancy.

✓ Correct Answer: D. Past cesarean pregnancy