## **Core Concept**
Infants develop social and cognitive skills at specific ages, one of which is stranger anxiety or the ability to discriminate strangers. This is a key developmental milestone.

## **Why the Correct Answer is Right**
Infants typically begin to show stranger anxiety around 6 to 9 months of age. This is a period where they develop object permanence and start to form attachments to primary caregivers. The ability to discriminate between familiar and unfamiliar faces or individuals is crucial for their social and emotional development. By 9 months, infants often exhibit clear signs of stranger anxiety, indicating they can differentiate between known and unknown people.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Infants at this age are just beginning to discover object permanence and are unlikely to exhibit complex social behaviors like stranger discrimination.
- **Option B:** While infants are making significant cognitive and social progress, 6 months might be a bit early for consistent stranger discrimination.
- **Option D:** This age is later than when stranger anxiety typically begins, though the ability to discriminate strangers would have been established.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that stranger anxiety peaks around 12 to 18 months of age. However, the ability to discriminate strangers starts a bit earlier, around 9 months. This developmental milestone is crucial for assessing infant development.

## **Correct Answer:** C. 9 months.