**Core Concept**
Uterine malformations can be a significant cause of recurrent abortions due to impaired placentation and altered uterine blood flow. The most common uterine malformation is characterized by a septate or bicornuate uterus, but the specific type is crucial for diagnosis.

**Why the Correct Answer is Right**
The correct answer is a septate uterus, which is a type of congenital uterine anomaly where the uterus is partially or completely divided by a thin wall of tissue. This malformation can lead to impaired placentation, recurrent miscarriages, and infertility. The septum can be easily identified on ultrasound, and hysteroscopy is often used to confirm the diagnosis.

**Why Each Wrong Option is Incorrect**
* **Option A:** A bicornuate uterus is characterized by a cleft in the upper part of the uterus, which is not the most common cause of recurrent abortions.
* **Option B:** A unicornuate uterus is a rare congenital anomaly where one side of the uterus is underdeveloped, which can cause recurrent miscarriages but is not the most common cause.
* **Option D:** A didelphic uterus is a rare congenital anomaly where the uterus is completely divided into two separate uteri, which is not a common cause of recurrent abortions.

**Clinical Pearl / High-Yield Fact**
A septate uterus is the most common cause of recurrent abortions, and hysteroscopic resection of the septum can significantly improve pregnancy outcomes.

**Correct Answer:** B.