**Core Concept**
The varicella-zoster virus (VZV) is a highly contagious and potentially severe infection in pregnant women, especially during the first trimester. The risk of congenital varicella syndrome is highest when maternal infection occurs between 13 and 20 weeks of gestation.

**Why the Correct Answer is Right**
The child can meet her aunt when the varicella rash has crusted over, which typically occurs 5-7 days after the onset of the rash. This is because the child is no longer contagious once the rash has crusted over, thereby minimizing the risk of transmitting the virus to her pregnant aunt. The crusting of the rash indicates that the child has developed a sufficient immune response to the virus, making her less likely to spread it to others.

**Why Each Wrong Option is Incorrect**
* **Option A:** The child can meet her aunt as soon as the rash appears, which is incorrect because the child is still highly contagious during this period.
* **Option B:** The child can meet her aunt 14 days after the onset of the rash, which is incorrect because this is a general guideline for isolation, but not specific to the risk of congenital varicella syndrome.
* **Option D:** The child can meet her aunt after 21 days of the onset of the rash, which is incorrect because the risk of congenital varicella syndrome is highest between 13 and 20 weeks of gestation, and the aunt's pregnancy status is not relevant to the child's contagious period.

**Clinical Pearl / High-Yield Fact**
The crusting of the varicella rash is a key indicator that the child is no longer contagious, and it is essential to wait at least 5-7 days after the onset of the rash before allowing the child to interact with pregnant individuals.

**Correct Answer:** C.