**Core Concept**
Transvaginal ultrasonography (USG) is a diagnostic tool used to visualize the female reproductive organs, including the gestation sac during early pregnancy. The gestation sac is a critical structure that forms after implantation of the blastocyst, and its detection is essential for confirming early pregnancy.

**Why the Correct Answer is Right**
The gestation sac is visible on transvaginal USG as early as 4.5-5 weeks of gestation. This is because transvaginal USG provides higher resolution images compared to transabdominal USG, allowing for earlier detection of the gestation sac. The gestation sac is a fluid-filled structure that forms around the embryo, and its presence is a key indicator of a viable pregnancy.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because the gestation sac is not visible on USG before 4.5-5 weeks of gestation. While some studies suggest that the yolk sac may be visible as early as 4.2 weeks, the gestation sac itself is not detectable until 4.5-5 weeks.
* **Option B:** This is incorrect because the embryo itself is not visible on USG until around 5-6 weeks of gestation. The embryo is a smaller structure compared to the gestation sac, and it takes higher resolution images to visualize it.
* **Option C:** This is incorrect because the gestation sac is not visible on USG before implantation, which occurs around 6-7 days after fertilization. The gestation sac forms after implantation, and its detection is a later event in early pregnancy.

**Clinical Pearl / High-Yield Fact**
Transvaginal USG is a crucial diagnostic tool for confirming early pregnancy and monitoring fetal development. It is essential to use high-frequency transducers and optimize the USG machine settings to obtain clear images of the gestation sac and embryo.

**Correct Answer:** D.