**Core Concept**
The patient's presentation of pelvic pain, low back pain, and a sensation of "pulling" or "stretching" in her groin, combined with a firm mass in the right adnexa and fluid in the abdominal and thoracic cavities, suggests a condition involving the rupture of an ovarian structure, leading to leakage of its contents into the surrounding spaces.

**Why the Correct Answer is Right**
The patient's symptoms and ultrasound findings are most consistent with a diagnosis of ovarian torsion with rupture, leading to hemoperitoneum and possibly a secondary pneumothorax or pneumoperitoneum. Ovarian torsion occurs when the ovary becomes twisted, compromising its blood supply and leading to ischemia and potentially rupture. The presence of fluid in the abdominal and thoracic cavities suggests rupture of the ovarian structure, with leakage of its contents into these spaces. This condition requires prompt surgical intervention to prevent further complications.

**Why Each Wrong Option is Incorrect**
**Option A:** Ruptured ectopic pregnancy is a possible diagnosis in a woman of childbearing age presenting with pelvic pain, but it would not typically present with a firm mass in the adnexa or fluid in the thoracic cavity.
**Option B:** Ovarian cyst rupture would typically present with acute pelvic pain and possibly some vaginal bleeding, but it would not typically involve the thoracic cavity.
**Option C:** Endometriosis can cause pelvic pain and a firm mass in the adnexa, but it would not typically involve fluid in the abdominal and thoracic cavities.

**Clinical Pearl / High-Yield Fact**
A classic presentation of ovarian torsion is the "triangle sign," where the twisted ovary and its associated structures form a triangular shape on ultrasound.

**Correct Answer:** C. Ovarian torsion with rupture.