**Core Concept**
The diagnosis of ectopic pregnancy involves identifying the location of the pregnancy and determining whether it is intrauterine or extrauterine. The modern diagnostic aid to diagnose ectopic pregnancy utilizes ultrasound imaging and beta-hCG levels to determine the presence and location of the pregnancy.

**Why the Correct Answer is Right**
The correct answer is based on the use of transvaginal ultrasound (TVUS) to visualize the adnexal region and detect an ectopic pregnancy. TVUS is particularly useful in the early detection of ectopic pregnancies, as it allows for the visualization of the ovaries, fallopian tubes, and other adnexal structures. The beta-hCG level is also an important diagnostic tool, as it can help to distinguish between an intrauterine and ectopic pregnancy. A lower beta-hCG level than expected for the gestational age is often indicative of an ectopic pregnancy.

**Why Each Wrong Option is Incorrect**
**Option A:** Hysterosalpingography (HSG) is an outdated method for diagnosing ectopic pregnancy, as it involves the injection of a contrast agent into the uterine cavity and can be associated with complications such as infection and pelvic pain.

**Option B:** Laparoscopy is a surgical procedure that is used to diagnose and treat ectopic pregnancies, but it is not a diagnostic aid in the classical sense.

**Option C:** Endovaginal ultrasound is a type of ultrasound imaging that is used to visualize the pelvic organs, but it is not as sensitive as transvaginal ultrasound for detecting ectopic pregnancies.

**Clinical Pearl / High-Yield Fact**
A beta-hCG level that is lower than expected for the gestational age, combined with the presence of an adnexal mass on ultrasound, is highly suggestive of an ectopic pregnancy.

**Correct Answer:** C. Endovaginal ultrasound