**Core Concept**
Placenta previa is a condition characterized by the placenta partially or completely covering the internal cervical os, which can lead to severe bleeding during pregnancy. Accurate diagnosis of the degree of placenta previa is crucial for determining the risk of bleeding and planning appropriate management.

**Why the Correct Answer is Right**
The best way to diagnose the degree of placenta previa is through ultrasound examination, specifically the transvaginal ultrasound. This modality provides a clear visualization of the placental edge in relation to the internal cervical os, allowing for accurate measurement of the distance between the placental edge and the os. The placental edge is considered to be in the internal cervical os when it is within 2 cm of the os.

**Why Each Wrong Option is Incorrect**
**Option A:** Physical examination, although useful for assessing the degree of cervical dilation and bleeding, is not reliable for diagnosing the degree of placenta previa due to the risk of provoking bleeding.

**Option B:** Color Doppler ultrasound can be used to assess blood flow in the placenta and fetus, but it is not the best modality for diagnosing the degree of placenta previa.

**Option C:** Magnetic Resonance Imaging (MRI) can provide detailed images of the placenta and uterus, but it is not the preferred modality for diagnosing placenta previa due to its limited availability and higher cost.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that transvaginal ultrasound is the best modality for diagnosing placenta previa, and it should be performed in all women with suspected placenta previa to accurately assess the degree of placental invasion.

**Correct Answer:** C. Transvaginal ultrasound