**Core Concept**
The investigation of choice in a diabetic mother with a doubtful abnormal fetus is a diagnostic tool that helps assess fetal well-being and detect any potential congenital anomalies. This is crucial in diabetic pregnancies, as they are associated with an increased risk of fetal anomalies and complications.

**Why the Correct Answer is Right**
In diabetic pregnancies, the preferred investigation is the **Non-Stress Test (NST)**. The NST is a non-invasive test that measures fetal heart rate in response to fetal movements. It is a reassuring test when the fetal heart rate accelerates in response to fetal movements, indicating fetal well-being. The NST is also a useful screening tool for detecting potential fetal distress or anomalies. The mechanism behind the NST involves the evaluation of fetal heart rate variability and accelerations, which are indicative of a healthy fetus.

**Why Each Wrong Option is Incorrect**
**Option A:** **Magnetic Resonance Imaging (MRI)** is not the investigation of choice in this scenario. While MRI can provide detailed images of the fetus, it is not a primary screening tool for detecting fetal anomalies in diabetic pregnancies.

**Option B:** **Ultrasound** is a useful tool in assessing fetal anatomy and growth, but it is not the specific investigation of choice for detecting fetal well-being in diabetic pregnancies.

**Option C:** **Biophysical Profile (BPP)** is a more comprehensive test that evaluates fetal well-being through a combination of ultrasound and non-stress testing. However, it is not the primary investigation of choice in diabetic pregnancies.

**Clinical Pearl / High-Yield Fact**
In diabetic pregnancies, it is essential to perform regular non-stress tests to monitor fetal well-being and detect any potential anomalies or complications. A normal non-stress test result is reassuring, but a non-reactive test may require further evaluation with a biophysical profile or other diagnostic tests.

**Correct Answer: A. Non-Stress Test (NST)**