**Core Concept**
Fast breathing rate is a key indicator for diagnosing pneumonia in children. The World Health Organization (WHO) provides guidelines for classifying respiratory rates in children based on age. These guidelines help clinicians diagnose pneumonia and provide timely treatment.

**Why the Correct Answer is Right**
In children aged 1-5 years, a respiratory rate of more than 40 breaths per minute is considered fast and indicative of pneumonia. This is because children in this age group have a higher metabolic rate and are more susceptible to respiratory infections. The fast breathing rate is a result of the child's attempt to compensate for the decreased oxygen levels in the blood.

**Why Each Wrong Option is Incorrect**
**Option A:** 30 breaths per minute - This is considered a normal respiratory rate for a two-year-old child.
**Option B:** 50 breaths per minute - While a higher respiratory rate, this is not the threshold for diagnosing pneumonia in children aged 1-5 years.
**Option C:** 45 breaths per minute - This is close to the correct answer but does not meet the WHO criteria for pneumonia in children.

**Clinical Pearl / High-Yield Fact**
Remember the WHO guidelines for respiratory rates in children: less than 40 breaths per minute for children aged 1-5 years is considered normal, while more than 40 breaths per minute is indicative of pneumonia.

**Correct Answer: C. 40 breaths per minute.**