**Core Concept**
The visible yolk sac is a crucial landmark in early fetal development, marking the transition from embryonic to fetal life. It is a fluid-filled structure that provides nutrients to the developing embryo before the placenta forms.

**Why the Correct Answer is Right**
The yolk sac is visible on ultrasound (USG) when it contains a significant amount of fluid, typically around 5-6 weeks of gestation. This is because the yolk sac starts to form around 2-3 weeks after fertilization and grows in size as the embryo develops. The visible yolk sac on USG is an important indicator of normal fetal development and is often used to confirm the gestational age.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the yolk sac is not visible on USG at this early stage of development. The embryo is still in the implantation phase, and the yolk sac has not yet formed.
* **Option B:** This option is incorrect because the yolk sac is visible on USG before this gestational age. The correct timeline is around 5-6 weeks, not 7-8 weeks.
* **Option C:** This option is incorrect because the yolk sac is not visible on USG at this late stage of development. The yolk sac typically disappears by around 8-10 weeks as the placenta takes over its function.

**Clinical Pearl / High-Yield Fact**
Remember that the visible yolk sac on USG is a key marker of normal fetal development and is often used to confirm the gestational age. However, it's essential to note that a small or absent yolk sac does not necessarily indicate a problem, as it may be due to various reasons such as fetal demise or a missed abortion.

**Correct Answer:** C.