## Core Concept
Infantile colic is a condition characterized by episodes of crying, often accompanied by symptoms such as drawing up of the legs, passing gas, and a hard, red face. It is a common condition affecting infants, typically caused by gastrointestinal issues, food sensitivities, or immaturity of the infant's digestive system.

## Why the Correct Answer is Right
The correct answer, **0-3 months**, is the most common age group affected by infantile colic because this condition typically peaks around 2-4 weeks of age and often resolves by 3-4 months of age. This timing aligns with the developmental stage of the infant's gastrointestinal tract and potential exposure to cow's milk proteins if formula-fed or through the breast milk of a mother consuming dairy products.

## Why Each Wrong Option is Incorrect
* **Option A: 6-9 months** - This age group is less commonly associated with infantile colic as the condition usually resolves by 3-4 months of age.
* **Option B: 3-6 months** - Although some cases might persist or start around this period, it's not the peak age for infantile colic onset.
* **Option D: 9-12 months** - At this age, infants are less likely to experience infantile colic as the condition typically resolves much earlier.

## Clinical Pearl / High-Yield Fact
A key point to remember is the "rule of threes" for infantile colic: episodes last more than 3 hours, occur more than 3 days per week, and typically start in the first 3 months of life. This helps in distinguishing colic from other causes of infant distress.

## Correct Answer: B. 0-3 months.