**Core Concept**
Ectopic pregnancy is a life-threatening condition where the embryo implants outside the uterus, most commonly in the fallopian tube. The site of implantation is critical in determining the management and prognosis of the condition.

**Why the Correct Answer is Right**
The rarity and commonality of ectopic pregnancy sites can be attributed to the anatomy of the female reproductive system. The fallopian tube is the most common site due to its proximity to the uterus and the path of the fertilized ovum. In contrast, implantation in the cervix is extremely rare due to its location and the absence of a suitable environment for embryonic development.

**Why Each Wrong Option is Incorrect**
**Option A:** Interstitial portion of the fallopian tube - While this is a possible site for ectopic pregnancy, it is not the rarest or most common site.
**Option B:** Cervix - Ectopic pregnancy in the cervix is extremely rare due to its location and the absence of a suitable environment for embryonic development.
**Option C:** Ovary - Ectopic pregnancy in the ovary is rare, but more common than in the cervix.

**Clinical Pearl / High-Yield Fact**
Remember that ectopic pregnancy is a medical emergency, and early diagnosis is critical to prevent maternal mortality. The classic triad of symptoms includes pelvic pain, vaginal bleeding, and amenorrhea.

**Correct Answer:** B. Cervix