Step 1: An ectopic pregnancy is implantation of the conceptus outside the uterine cavity. The site distribution is heavily skewed towards the fallopian tube.
Step 2: The fallopian tube is by far the most common location for an ectopic gestation, accounting for around 90 to 97 percent of all cases. From the given choices, 90% is the best match, so option (b) is correct.
Step 3: Within the tube the ampulla is the commonest segment, followed by the isthmus, with the interstitial and fimbrial portions being rarer.
Step 4: The lower values offered (67%, 75%, 80%) understate the true tubal predominance and are therefore incorrect, while non-tubal sites such as the ovary, cervix and abdomen together make up only a small minority.