**Core Concept**
Placenta previa is a condition where the placenta partially or totally covers the internal cervical os, leading to potential complications such as severe vaginal bleeding. Conservative management may be considered in specific cases where the risk of bleeding is low.

**Why the Correct Answer is Right**
In cases of low-lying placenta previa, conservative management may be possible if the placenta is not covering the internal cervical os, and the bleeding is minimal. This is because the risk of severe bleeding is low, and the patient can be closely monitored. The key factor is that the placenta is not covering the cervical os, which reduces the risk of bleeding. The management involves bed rest, frequent ultrasound monitoring, and blood transfusions as needed.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because conservative management is not usually possible in cases of placenta previa where the patient is experiencing severe bleeding. Severe bleeding is a contraindication for conservative management.

**Option B:** This option is incorrect because conservative management is not possible in cases where the placenta is covering the internal cervical os, as this increases the risk of severe bleeding.

**Option C:** This option is incorrect because conservative management is not possible in cases where the patient is experiencing preterm labor or has other complications such as fetal distress. These situations require immediate delivery.

**Option D:** This option is incorrect because conservative management is not possible in cases where the patient has a history of placenta previa in a previous pregnancy. This increases the risk of complications and requires immediate delivery.

**Clinical Pearl / High-Yield Fact**
In cases of placenta previa, it's essential to differentiate between low-lying placenta and placenta previa. Low-lying placenta means the placenta is close to the cervical os, but not covering it. Placenta previa means the placenta is covering the cervical os, which increases the risk of bleeding.

**Correct Answer:** B.