**Core Concept**
The question requires the identification of the stage of labor based on cervical dilatation and contraction rate. Labor is divided into four stages: the first stage of labor (from the onset of true labor to full cervical dilatation), the second stage of labor (from full cervical dilatation to the delivery of the baby), the third stage of labor (from the delivery of the baby to the delivery of the placenta), and the fourth stage of labor (from the delivery of the placenta to the return of the uterus to its normal size).

**Why the Correct Answer is Right**
A primigravida at 6 cm cervical dilatation is in the active phase of the first stage of labor. The active phase of labor is characterized by a contraction rate of 2-5 contractions per 10 minutes, with each contraction lasting 45-60 seconds and resulting in a cervical dilatation of 1 cm or more per hour. The contraction rate of 3/10 minutes is within the normal range, and the cervical dilatation of 6 cm indicates that the patient is in the active phase of the first stage of labor.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a cervical dilatation of 6 cm is not characteristic of the latent phase of labor, which is the initial phase of labor where the cervix dilates slowly.

**Option B:** This option is incorrect because a cervical dilatation of 6 cm is not characteristic of the second stage of labor, which begins after the cervix is fully dilated.

**Option C:** This option is incorrect because a cervical dilatation of 6 cm is not characteristic of the third stage of labor, which begins after the delivery of the baby.

**Option D:** This option is incorrect because a cervical dilatation of 6 cm is not characteristic of the fourth stage of labor, which begins after the delivery of the placenta.

**Clinical Pearl / High-Yield Fact**
To determine the stage of labor, remember the mnemonic "DCBA": Dilatation, Contraction rate, Baby, and After. This mnemonic helps to remember that the stage of labor is determined by the cervical dilatation, contraction rate, the presence of the baby's head in the birth canal, and the time after delivery.

**Correct Answer: B. First stage of labor.**