Okay, let's tackle this question. The user is asking about the age group selected by WHO for immunization surveys. I need to recall the WHO guidelines. Wait, I remember that WHO often uses a specific age group to assess immunization coverage, like children aged 12-23 months. That's because this group has had enough time to receive all the primary vaccines but before any booster doses, making it a standard for measuring coverage.

The core concept here is understanding the methodology WHO uses in their surveys. The correct answer is likely 12-23 months. The other options might be different age ranges, like 6-11 months or 24-36 months. Let me think why those are incorrect. Younger children might not have completed their vaccination schedule, while older ones might have received boosters, skewing the results.

Clinical pearls: The key point is that 12-23 months is the standard age group for assessing immunization coverage. This helps in evaluating the effectiveness of the primary vaccination series. Students should remember this age range for WHO surveys.

**Core Concept**
The World Health Organization (WHO) uses a standardized age group to assess immunization coverage, ensuring consistency in evaluating vaccine administration and identifying gaps in public health programs. This age group is chosen to capture children who have received all primary vaccine doses but not yet boosters.

**Why the Correct Answer is Right**
The WHO survey targets children aged **12–23 months**. This group has completed the primary vaccination schedule (e.g., BCG, DTP, OPV, measles) but has not yet received booster doses, making it ideal for measuring baseline immunization coverage without confounding variables. This age range ensures data reflects the efficacy of the initial vaccination program.

**Why Each Wrong Option is Incorrect**
**Option A: <12 months** – Incorrect. Younger children may not have completed all primary vaccines, leading to underestimation of coverage. **Option B: 24–36 months** – Incorrect. Older children may have received booster doses, which could inflate coverage statistics and obscure gaps in primary series completion. **Option C: 3–5 years** – Incorrect. This age group includes children who might have missed vaccines entirely or received them irregularly, reducing survey reliability. **Clinical Pearl / High-Yield Fact** **12–23 months** is the WHO-recommended age group for immunization surveys. Remember this range for exams; it is critical for assessing vaccine-preventable disease control and planning public health interventions. **Correct Answer: C. 12–23 months**

✓ Correct Answer: A. 12 - 23 months