**Core Concept**
Active labor in a primigravida is characterized by a predictable and progressive cervical dilatation, which is a crucial indicator of labor progression. This process is influenced by the uterine contractions and the softening and effacement of the cervix.

**Why the Correct Answer is Right**
The correct rate of cervical dilatation in a primigravida during active labor is approximately 1 cm/hour. This is because the cervix undergoes significant changes, including softening and effacement, which facilitate the dilatation process. The uterine contractions play a crucial role in this process by exerting a constant and progressive pressure on the cervix, leading to its gradual dilatation.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because the rate of cervical dilatation in a primigravida during active labor is not typically less than 0.5 cm/hour.

**Option B:**
This option is incorrect because the rate of cervical dilatation in a primigravida during active labor is not typically greater than 2 cm/hour.

**Option D:**
This option is incorrect because the rate of cervical dilatation in a primigravida during active labor is not typically 1.5 cm/hour.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the rate of cervical dilatation in a primigravida during active labor can vary depending on various factors, including the presence of any underlying medical conditions and the effectiveness of uterine contractions.

**Correct Answer:** B. 1 cm/hour.