**Core Concept**
Molar pregnancy occurs when a non-viable fertilized egg implants in the uterus and develops abnormally, leading to an excessive production of human chorionic gonadotropin (hCG) hormone. This condition is characterized by an abnormal proliferation of trophoblastic cells, which are the cells that normally give rise to the placenta.

**Why the Correct Answer is Right**
The diagnosis of molar pregnancy is typically made by ultrasonography, which reveals characteristic features such as an empty gestational sac, absence of fetal cardiac activity, and the presence of hyperechogenic masses suggestive of trophoblastic proliferation. The high levels of hCG in molar pregnancy also lead to an exaggerated uterine enlargement, which can be detected on clinical examination. The combination of ultrasonographic findings and high hCG levels is diagnostic of molar pregnancy.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because molar pregnancy is not typically diagnosed by serum beta hCG levels alone, although high levels can be a clue. Other conditions such as ectopic pregnancy or multiple pregnancy can also present with high hCG levels.
**Option B:** This option is incorrect because a molar pregnancy is not diagnosed by the presence of a fetal cardiac activity on ultrasound. In fact, the absence of fetal cardiac activity is a key feature of molar pregnancy.
**Option C:** This option is incorrect because a molar pregnancy is not diagnosed by the presence of uterine contractions alone. While uterine contractions can be a feature of molar pregnancy, they are not diagnostic.

**Clinical Pearl / High-Yield Fact**
A molar pregnancy should always be suspected in a woman who presents with an exaggerated uterine enlargement and high hCG levels, and the diagnosis should be confirmed by ultrasonography to avoid unnecessary surgical interventions.

**Correct Answer: A. Ultrasonography.**