D is the correct answer

Various screening tests exist for fetal aneuploidy, defined as more or less than 46 chromosomes. The most common types of aneuploidy are trisomies 13 (Patau syndrome), 18 (Edwards syndrome), and 21 (Down syndrome). First-trimester genetic screening can be done using maternal DNA markers, maternal hormone levels, and ultrasonography. The first-trimester ultrasound screen measures nuchal translucency, or the hypoechoic pocket that is just ventral to the dorsal neck. This is used in combination with human chorionic growth hormone and pregnancy-associated plasma protein A levels to predict the risk of aneuploidy. Second-trimester ultrasound can show obvious structural abnormalities as well as soft markers for aneuploidy. Of the soft markers listed above, a thickened nuchal skin fold as an isolated finding confers the greatest risk for trisomy 21.

Atrial septal defect (A) is an anomaly seen in a variety of genetic conditions, as well as in isolation. Small atrial septal defects may resolve on their own. Therefore, this would not be a specific marker for Down syndrome. Echogenic bowel (B) appears as a bright spot in the abdomen of the fetus on ultrasound. Although it may be associated with Down syndrome, it is also associated with the other trisomies. In the majority of cases, echogenic bowel is an incidental and benign finding. Echogenic intracardiac focus (C), in isolation, confers the least specific risk for fetal aneuploidy. Often, these resolve on subsequent scans.