**Core Concept**
The **latent phase** of labor refers to the initial period where the cervix dilates from 0 to 3-4 cm. Prolonged latent phase is a concern as it may indicate issues with labor progression.

**Why the Correct Answer is Right**
A prolonged latent phase in a **nullipara** (a woman giving birth for the first time) is generally defined as one that exceeds 20 hours. This definition is based on the understanding that labor in nulliparous women tends to be longer than in multiparous women.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because 8 hours is too short to be considered a prolonged latent phase, even in multiparous women.
**Option B:** This option is also incorrect as 12 hours, while potentially concerning, does not meet the criteria for a prolonged latent phase in nulliparas.
**Option D:** This option is incorrect as 24 hours might be considered prolonged but the specific definition for nulliparas is less than this.

**Clinical Pearl / High-Yield Fact**
It's crucial to distinguish between the latent and active phases of labor, as management strategies differ significantly. A prolonged latent phase may require augmentation or other interventions to ensure a safe delivery.

**Correct Answer:** C. 20 hours.