**Core Concept**
The question is testing the ability to identify a sonographic finding that confirms an intrauterine pregnancy. This involves understanding the ultrasonographic characteristics of a normal intrauterine pregnancy and differentiating it from other causes of a positive pregnancy test.

**Why the Correct Answer is Right**
The correct answer is the sonographic finding that is most consistent with an intrauterine pregnancy. Intrauterine pregnancy is characterized by the presence of a gestational sac, an embryo, and a yolk sac within the uterine cavity. The presence of a yolk sac within the gestational sac is a key feature that confirms an intrauterine pregnancy. The yolk sac is a membrane-like structure that surrounds the embryo and provides it with nutrients.

**Why Each Wrong Option is Incorrect**
* **Option A:** The presence of a corpus luteum cyst alone does not confirm an intrauterine pregnancy. Corpus luteum cysts can be found in the ovaries of women who are not pregnant.
* **Option B:** A positive pregnancy test does not confirm an intrauterine pregnancy. Pregnancy tests detect the presence of human chorionic gonadotropin (hCG) in the urine or blood, which can be present in ectopic pregnancies or false positives.
* **Option D:** A small amount of free fluid in the pelvis can be seen in various conditions, including ovarian cysts, endometriosis, or ectopic pregnancy. It does not confirm an intrauterine pregnancy.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the presence of a yolk sac within the gestational sac is a key feature that confirms an intrauterine pregnancy. However, the absence of a yolk sac does not necessarily rule out an intrauterine pregnancy, as it may not be visualized due to its small size or location.

**Correct Answer:** C.