**Core Concept**
Pomeroy's method is a popular technique for female sterilization, which involves ligating a portion of the Fallopian tube to prevent fertilization. This procedure is a form of tubal ligation.

**Why the Correct Answer is Right**
During Pomeroy's method, the ampullary portion of the Fallopian tube is ligated. The ampulla is the widest portion of the tube, where fertilization typically occurs. Ligation at this site effectively prevents the egg from traveling to the uterus, thereby preventing fertilization and pregnancy. The procedure involves making a small incision in the ampulla, followed by ligation using a thread or clip.

**Why Each Wrong Option is Incorrect**
* **Option A:** The **infundibulum** is the funnel-shaped portion of the tube that contains fimbriae, which pick up the ovum from the ovary. Ligation at this site would not effectively prevent fertilization.
* **Option B:** The **isthmus** is the narrow portion of the tube that connects the ampulla to the uterus. While ligation at this site might prevent fertilization, it is not the preferred site for Pomeroy's method due to the risk of damaging the uterine cavity.
* **Option C:** The **interstitial portion** of the tube is the portion that passes through the uterine wall. Ligation at this site would not be practical or effective for sterilization.

**Clinical Pearl / High-Yield Fact**
Pomeroy's method is a widely used and effective technique for female sterilization, with a high success rate. However, it's essential to note that sterilization is a permanent method of contraception and should be considered carefully before undergoing the procedure.

**Correct Answer: C. The ampullary portion of the Fallopian tube is ligated.**