Major pa of cardiac septa development is completed by day:

**Core Concept**
Cardiac septation is a critical process in embryonic development, where the heart divides into separate chambers and a septum forms to separate the left and right sides of the heart. This process is essential for the proper development of the heart and its function.

**Why the Correct Answer is Right**
The major part of cardiac septation is completed by 8 weeks of gestation. During this period, the septum primum and septum intermedium fuse to form the atrial septum, while the endocardial cushions and the muscular septum fuse to form the ventricular septum. The formation of the atrioventricular septum also occurs during this period. This septation process is crucial for the proper development of the heart and its function.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because cardiac septation is not completed by day 28 of gestation. While some initial septation may occur during this period, the major part of septation is still to come.

**Option B:** This option is incorrect because cardiac septation is not completed by day 56 of gestation. While some refinement of the septa may occur during this period, the major part of septation was completed earlier.

**Option C:** This option is incorrect because cardiac septation is not completed by day 70 of gestation. While some final touches to the septa may occur during this period, the major part of septation was completed earlier.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that cardiac septation is a complex process that requires precise timing and coordination. Any disruption to this process can lead to congenital heart defects, which are a leading cause of morbidity and mortality in infants and children.

**Correct Answer:** D. 8 weeks.