**Core Concept**
Sex determination in early pregnancy is crucial for prenatal diagnosis and genetic counseling. Ultrasound is a non-invasive imaging technique that uses high-frequency sound waves to visualize the fetus. It is commonly used for sex determination between 16 and 20 weeks of gestation.

**Why the Correct Answer is Right**
Ultrasound is the preferred method for sex determination due to its safety and accuracy. During an ultrasound examination, the fetal genitalia can be visualized, and the presence of a penis or clitoris can be used to determine the sex. This is possible because the genitalia of a male fetus are more easily distinguishable from those of a female fetus. The ultrasound technician or obstetrician can use this information to determine the sex of the fetus.

**Why Each Wrong Option is Incorrect**
* **Option A:** X-rays are not used for sex determination in early pregnancy because they involve radiation, which can be harmful to the fetus. Additionally, X-rays do not provide sufficient resolution to visualize the fetal genitalia.
* **Option B:** Amniocentesis is a diagnostic procedure that involves withdrawing a sample of amniotic fluid from the womb to test for genetic disorders. While it can provide information about the sex of the fetus, it is typically performed between 15 and 20 weeks of gestation and carries a small risk of miscarriage.
* **Option D:** Hysteroscopy is a procedure that involves inserting a scope into the uterus to visualize the uterine cavity. It is not used for sex determination in early pregnancy and is typically used to diagnose uterine abnormalities or to perform surgical procedures.

**Clinical Pearl / High-Yield Fact**
The accuracy of sex determination by ultrasound increases with gestational age, and it is recommended to perform the examination between 16 and 20 weeks of gestation. Before this period, the genitalia may not be fully developed, and the accuracy of sex determination may be lower.

**✓ Correct Answer: C. Ultrasound**