**Core Concept**
The earliest detection of a gestational sac using ultrasonography is a critical milestone in early pregnancy diagnosis. It marks the beginning of the embryonic development and is essential for assessing gestational age and potential complications.

**Why the Correct Answer is Right**
The gestational sac is the first visible structure in early pregnancy, and its detection is crucial for confirming pregnancy and estimating gestational age. It appears as a small, anechoic (black) ring within the uterus, surrounded by a hyperechoic (white) rim. The sac can be seen as early as 4.5 to 5 weeks of gestation, and its presence is a reliable indicator of an intrauterine pregnancy. The detection of the gestational sac is made possible by the use of transvaginal ultrasonography, which provides higher resolution images of the uterus and its contents.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is too early for the gestational sac to be visible on ultrasonography. While implantation occurs around this time, the sac is not yet large enough to be detected.
**Option B:** This option is incorrect because it is within the range for the detection of the gestational sac, but it is not the earliest time at which it can be seen.
**Option C:** This option is incorrect because it is too late for the gestational sac to be visible on ultrasonography. By this time, the embryo has developed further and is more easily detectable.

**Clinical Pearl / High-Yield Fact**
In early pregnancy, the detection of a gestational sac on ultrasonography is a reliable indicator of an intrauterine pregnancy. However, it is essential to note that a negative ultrasound result does not rule out pregnancy, and a repeat scan may be necessary to confirm or exclude pregnancy.

**Correct Answer: C. 5 weeks**