**Core Concept**
The question is testing the association between maternal diabetes and congenital heart disease in the offspring. Maternal diabetes is a known risk factor for congenital anomalies, and cardiac malformations are a common type of anomaly in diabetic pregnancies.

**Why the Correct Answer is Right**
The most likely diagnosis in this scenario is Trisomy 21, also known as Down syndrome. Maternal diabetes is a known risk factor for Trisomy 21, and cardiac malformations are a common feature of this condition. The presence of a cardiac malformation in the newborn, combined with the maternal history of diabetes, makes Trisomy 21 a strong possibility. Trisomy 21 is caused by an extra copy of chromosome 21, which leads to a range of clinical features, including cardiac anomalies, developmental delay, and characteristic facial features.

**Why Each Wrong Option is Incorrect**
* **Option A:** Turner syndrome is a genetic disorder that affects females, characterized by the absence of one X chromosome. While Turner syndrome can cause cardiac anomalies, it is not typically associated with maternal diabetes.
* **Option B:** Congenital rubella syndrome is a condition caused by maternal infection with rubella virus during pregnancy. While it can cause cardiac anomalies, it is not typically associated with maternal diabetes.
* **Option C:** DiGeorge syndrome is a genetic disorder that affects the development of the pharyngeal pouches and is characterized by cardiac anomalies, thymic hypoplasia, and hypocalcemia. While it can cause cardiac anomalies, it is not typically associated with maternal diabetes.

**Clinical Pearl / High-Yield Fact**
Maternal diabetes is a known risk factor for congenital anomalies, including cardiac malformations, and should be considered in the differential diagnosis of any newborn with a cardiac anomaly.

**Correct Answer: D. Trisomy 21. Down syndrome.**