**Core Concept**
Hysterosalpingogram (HSG) is a radiographic procedure used to assess the shape of the uterine cavity and the patency of the fallopian tubes. It involves the injection of a radiopaque contrast medium through the cervix, under fluoroscopic guidance, to visualize the uterine cavity and fallopian tubes.

**Why the Correct Answer is Right**
The optimal time for performing an HSG in a woman with a regular 28-day menstrual cycle is between days 7 to 10 of her cycle. This is because, during this period, the endometrial lining is thin, and the risk of bleeding and perforation is minimized. Additionally, the ovaries are typically in the follicular phase, and the risk of ovarian cysts is lower. The correct day is **Day 9** of a 28-day menstrual cycle, as it is the most optimal time for the procedure.

**Why Each Wrong Option is Incorrect**
**Option A:** Day 1 is incorrect because it is the first day of menses, and the endometrium is thick, increasing the risk of bleeding and perforation.
**Option B:** Day 14 is incorrect because it is the day of ovulation, and the risk of ovarian cysts is higher.
**Option C:** Day 21 is incorrect because it is the day of the luteal phase, and the endometrium is thick, increasing the risk of bleeding and perforation.

**Clinical Pearl / High-Yield Fact**
It is essential to perform HSG between days 7 to 10 of a woman's menstrual cycle to minimize the risk of complications and ensure optimal visualization of the uterine cavity and fallopian tubes.

**Correct Answer:** C. Day 9.