**Core Concept**
Fast breathing in infants is a critical indicator of respiratory distress or other underlying conditions. The World Health Organization (WHO) has established guidelines for assessing infant breathing rates, taking into account age-specific norms.

**Why the Correct Answer is Right**
For infants between 2-12 months, the WHO defines tachypnea (fast breathing) as a respiratory rate exceeding 50 breaths per minute. This threshold is based on the understanding that infants have a higher metabolic rate and heart rate compared to adults, and their respiratory rate is influenced by their age and overall health. In a 6-month-old infant, a respiratory rate above 50 breaths per minute may indicate respiratory distress, infection, or other conditions requiring medical attention.

**Why Each Wrong Option is Incorrect**
**Option A:** A respiratory rate of >60 breaths/minute is not the standard threshold for fast breathing in 6-month-old infants, as it is more commonly seen in younger or older infants.
**Option C:** A rate of >40 breaths/minute may be normal for a 6-month-old infant, and is not typically considered fast breathing.
**Option D:** A rate of >30 breaths/minute is well within the normal range for a 6-month-old infant and is not indicative of fast breathing.

**Clinical Pearl / High-Yield Fact**
It's essential to note that fast breathing in infants can be a non-specific symptom and may not always be associated with respiratory problems. However, it is crucial to assess the infant's overall condition, including other signs of respiratory distress, such as grunting, flaring, or cyanosis.

**✓ Correct Answer: B. >50 breaths/ min**