**Core Concept**
The ectopic pregnancy is a life-threatening condition where the fertilized ovum implants outside the uterine cavity, typically in the fallopian tubes. The frequency of ectopic implantation varies along the fallopian tube, with certain sections being more prone to ectopic pregnancies due to their anatomy and physiological characteristics.

**Why the Correct Answer is Right**
The ampulla is the widest part of the fallopian tube, with a larger diameter compared to the other sections. This increased diameter allows for a greater chance of implantation of the fertilized ovum. Additionally, the ampulla is closer to the ovary, where the ovum is released, making it easier for the ovum to travel and implant in this section. The ampulla's relatively slow peristalsis and larger diameter facilitate a longer residence time, increasing the likelihood of ectopic implantation.

**Why Each Wrong Option is Incorrect**
**Option A:** The interstitial portion of the fallopian tube, which traverses the uterine wall, has a smaller diameter and is less likely to be the site of ectopic implantation due to its narrower lumen and relatively faster peristalsis.

**Option B:** The isthmic portion of the fallopian tube, connecting the ampulla to the uterine cavity, has a smaller diameter and is less likely to be the site of ectopic implantation compared to the ampulla.

**Option D:** The infundibulum, the funnel-shaped portion of the fallopian tube closest to the ovary, is responsible for receiving the ovum from the ovary but is less likely to be the site of ectopic implantation due to its narrower diameter and faster peristalsis.

**Clinical Pearl / High-Yield Fact**
The risk of ectopic pregnancy is higher in women with a history of pelvic surgery, previous ectopic pregnancy, and certain medical conditions such as endometriosis or pelvic inflammatory disease.

**✓ Correct Answer:** C. Ampulla