**Core Concept**
The gestational sac is a fluid-filled structure that forms within the uterus during early pregnancy, serving as a protective environment for the developing embryo. Transvaginal ultrasound (TVS) is a diagnostic tool used to visualize the gestational sac and confirm pregnancy.

**Why the Correct Answer is Right**
The gestational sac can be visualized by TVS as early as 4.5-5 weeks of gestation, when the embryo is approximately 5-6 mm in size. This is because TVS provides high-resolution images of the uterus and its contents, allowing for the detection of the gestational sac and its characteristic ring of echoes. The gestational sac is visible due to the presence of yolk sac and embryonic pole, which are detectable by TVS.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the gestational sac can be visualized earlier than 5 weeks, as mentioned earlier.
**Option B:** This option is incorrect because the crown-rump length (CRL) of the embryo is a different parameter used to estimate gestational age, and it is not directly related to the visibility of the gestational sac.
**Option C:** This option is incorrect because the gestational sac is not a "soft marker" for Down syndrome, but rather a normal finding in early pregnancy.

**Clinical Pearl / High-Yield Fact**
The presence of a gestational sac on TVS is a critical finding in early pregnancy, as it confirms intrauterine pregnancy and helps to rule out ectopic pregnancy.

**Correct Answer: A. 4.5-5 weeks**