**Core Concept**
Active labor is characterized by the onset of regular uterine contractions that result in cervical dilatation and effacement. It is a critical phase of the labor process that requires precise monitoring and management to ensure a safe and successful delivery.

**Why the Correct Answer is Right**
The start of active labor is typically defined by the onset of regular uterine contractions that cause the cervix to dilate at a rate of 1 cm per hour. This is often accompanied by a change in fetal station and a decrease in fetal heart rate variability. The correct answer is based on the American College of Obstetricians and Gynecologists (ACOG) guidelines, which recommend that active labor is diagnosed when the cervix is 4-7 cm dilated.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify a clear criterion for the start of active labor. Cervical effacement alone is not a definitive indicator of active labor.
* **Option B:** This option is incorrect because it refers to the latent phase of labor, which is characterized by irregular uterine contractions and little or no cervical dilatation.
* **Option C:** This option is incorrect because it refers to a rate of cervical dilatation that is typical of the latent phase of labor, rather than active labor.

**Clinical Pearl / High-Yield Fact**
It is essential to distinguish between the latent and active phases of labor, as the management and interventions required during each phase differ significantly. A clear understanding of the criteria for the start of active labor is crucial for effective labor management.

**Correct Answer:** C. Cervix is 4-7 cm dilated.