**Core Concept**
The storage of vaccines at Primary Health Centers (PHCs) is crucial for maintaining their potency and effectiveness. Vaccines are biological products that require specific storage conditions to preserve their viability. The **cold chain** is essential for vaccine storage.

**Why the Correct Answer is Right**
Vaccines can be stored at PHCs for a limited period, typically in **cold storage facilities** with temperatures between 2°C and 8°C. This temperature range helps to slow down the degradation of the vaccine components. The specific duration of storage depends on the type of vaccine and the storage conditions.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the storage duration is not typically measured in such short periods.
**Option B:** Incorrect as it exceeds the typical storage limit for most vaccines at PHCs.
**Option C:** Incorrect because the duration is too short for effective vaccine storage and distribution.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that vaccines should always be stored in the **middle shelf of the refrigerator** to maintain a consistent temperature, and regular temperature monitoring is crucial to prevent vaccine degradation.

**Correct Answer:** D. One month or less, or as per the manufacturer's instructions and local guidelines.