**Core Concept**
The patient is presenting with painless vaginal bleeding in the third trimester, which is a classic symptom of placenta previa. This condition occurs when the placenta partially or completely covers the internal cervical os, leading to bleeding.

**Why the Correct Answer is Right**
The correct answer is based on the clinical presentation of painless vaginal bleeding in a pregnant woman at 38 weeks gestation with an engaged head and a non-tender, relaxed uterus. This scenario is highly suggestive of placenta previa. In such cases, the immediate goal is to ensure fetal and maternal safety, and the next line of treatment involves hospitalization and preparation for potential cesarean delivery.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because immediate delivery is not always necessary in cases of placenta previa, especially if the patient is stable and the fetus is mature.

**Option B:** This option is incorrect because manual removal of the placenta is a procedure typically performed after delivery, not before.

**Option C:** This option is incorrect because administration of oxytocin is contraindicated in cases of placenta previa, as it can exacerbate bleeding.

**Clinical Pearl / High-Yield Fact**
In cases of suspected placenta previa, it's essential to remember that the patient should be managed in a hospital setting, and preparations should be made for potential cesarean delivery.

**Correct Answer:** B. Bed rest in hospital.