**Core Concept**
Transvaginal ultrasound is a diagnostic imaging technique used to visualize the female reproductive organs, including the gestation sac in early pregnancy. The earliest detection of the gestation sac is crucial for confirming pregnancy and estimating gestational age.

**Why the Correct Answer is Right**
The gestation sac can be detected as early as 14 days after ovulation, which corresponds to 10 days post-fertilization. This is because the gestation sac starts to form around 6-8 days post-fertilization, and by 10-14 days, it is large enough to be visualized using transvaginal ultrasound. The sac typically appears as a small, anechoic (black) structure within the endometrial cavity, surrounded by a thin, hyperechoic (white) ring.

**Why Each Wrong Option is Incorrect**
**Option A:** 21 days after ovulation is too late for the earliest detection of the gestation sac, as it would correspond to around 17-18 days post-fertilization, when the embryo is already more developed and the sac is larger.

**Option B:** 21 days after implantation is not a relevant time frame, as implantation occurs around 6-8 days post-fertilization, and the gestation sac is detectable before implantation.

**Option C:** 28 days post-ovulation is equivalent to around 4-5 weeks of gestation, which is too late for the earliest detection of the gestation sac.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the earliest detection of the gestation sac on transvaginal ultrasound is a critical milestone in confirming pregnancy and estimating gestational age. A missed gestation sac or an empty uterine cavity can indicate a potential ectopic pregnancy or other complications.

**✓ Correct Answer: D. 14 days after ovulation. 14 days after ovulation.**