**Core Concept**
The patient's presentation with six weeks amenorrhoea and a mass abdomen with an empty uterus on ultrasound suggests an ectopic pregnancy with tubal rupture, but other causes like ovarian cysts, fibroids, or adnexal torsion should also be considered. In this scenario, the empty uterus points towards an extrauterine pregnancy.

**Why the Correct Answer is Right**
The correct diagnosis is a ruptured ectopic pregnancy, likely due to tubal rupture. This occurs when the fertilized ovum implants outside the uterus, most commonly in the fallopian tube. As the embryo grows, it can cause the tube to rupture, resulting in abdominal pain, hemoperitoneum, and potentially life-threatening bleeding. The empty uterus on ultrasound is a critical finding that supports this diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Ovarian cysts can cause abdominal masses, but they would typically be seen within the uterus on ultrasound, not empty.
**Option B:** Fibroids are benign tumors of the uterus, and their presence would be seen within the uterus on ultrasound, not empty.
**Option C:** Adnexal torsion can cause abdominal pain and a mass, but it would typically be associated with ovarian or tubal pathology, not an empty uterus.

**Clinical Pearl / High-Yield Fact**
In cases of suspected ectopic pregnancy, it's essential to consider the patient's last menstrual period, beta-hCG levels, and ultrasound findings to guide further management.

**Correct Answer:** C.