**Core Concept**
The early detection of fetal heart activity is crucial in obstetrics for assessing fetal viability and well-being. Fetal heart activity can be detected using various ultrasound techniques, with the earliest detection possible through the use of Doppler ultrasound.

**Why the Correct Answer is Right**
The earliest detectable sign of fetal heart activity is the _fetal cardiac activity_ that can be visualized using Doppler ultrasound as early as 5.5 weeks of gestation. This occurs due to the presence of the fetal heart beat, which is generated by the contraction and relaxation of the fetal cardiac muscle cells. The early detection of fetal heart activity is essential for determining fetal viability and the potential for ongoing pregnancy.

**Why Each Wrong Option is Incorrect**
**Option A:** 6 weeks - This is too late for the earliest detection of fetal heart activity, as it can be detected earlier using Doppler ultrasound.
**Option B:** 3 weeks - This is too early, as fetal heart activity cannot be detected at this gestational age.
**Option C:** 4 weeks - This is still too early for the earliest detection of fetal heart activity, as it can be detected earlier using Doppler ultrasound.

**Clinical Pearl / High-Yield Fact**
The earliest detectable sign of fetal heart activity is crucial for determining fetal viability and the potential for ongoing pregnancy. Fetal heart activity can be detected using Doppler ultrasound as early as 5.5 weeks of gestation.

**Correct Answer:** C. 5.5 weeks