**Core Concept**
Transvaginal ultrasound (USG) is a diagnostic tool used to detect early pregnancy, and the gestational sac is a critical landmark for confirming intrauterine pregnancy. The gestational sac is a fluid-filled structure that forms within the uterine cavity shortly after fertilization.

**Why the Correct Answer is Right**
The gestational sac can be detected as early as 4.5-5 weeks of gestation using transvaginal ultrasound. This is because transvaginal ultrasound provides a higher resolution image of the uterus and allows for a closer proximity to the uterine cavity, making it possible to detect the gestational sac at an earlier gestational age. The sac is visualized as an anechoic area within the uterine cavity, and its detection is an important milestone in confirming a viable intrauterine pregnancy.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not the earliest gestational age at which the gestational sac can be detected. While 6 weeks is a common gestational age for detecting the gestational sac, it is not the earliest possible time.
**Option B:** This option is incorrect because it is not a specific gestational age and does not provide a clear answer. A more specific answer is required to accurately determine the correct option.
**Option C:** This option is incorrect because it is not a valid gestational age for detecting the gestational sac. While 8 weeks is a common gestational age for detecting fetal cardiac activity, it is not the earliest possible time for detecting the gestational sac.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the detection of the gestational sac is an important milestone in confirming a viable intrauterine pregnancy, and its absence or presence should be carefully evaluated in the context of the patient's symptoms and clinical history.

**Correct Answer:** C. 5 weeks