**Core Concept**
Cardiotocography (CTG) is a monitoring technique that combines fetal heart rate (FHR) and uterine contractions to assess fetal well-being during labor. Fetal hypoxia, or decreased oxygen supply, can lead to changes in the FHR pattern, which can be detected through CTG.

**Why the Correct Answer is Right**
The most significant finding in CTG for detection of fetal hypoxia is the presence of late decelerations. Late decelerations occur when the FHR decreases in amplitude after the peak of a uterine contraction, indicating that the fetus is not receiving adequate oxygen supply. This is because the fetal heart rate is responding to the decreased oxygen levels in the fetal blood. The vagus nerve, which is responsible for regulating the FHR, responds to the decreased oxygen levels by causing a decrease in heart rate.

**Why Each Wrong Option is Incorrect**
* **Option A:** Early decelerations are a normal response to fetal head compression during uterine contractions and are not indicative of fetal hypoxia.
* **Option B:** Variable decelerations can be caused by umbilical cord compression or other factors, but are not the most significant finding for detecting fetal hypoxia.
* **Option D:** Tachycardia, or an elevated FHR, can be a sign of fetal distress, but is not the most specific indicator of hypoxia.

**Clinical Pearl / High-Yield Fact**
Remember that late decelerations are a more reliable indicator of fetal hypoxia than early decelerations or variable decelerations, and should prompt further investigation and intervention to ensure fetal well-being.

**Correct Answer:** C.