**Core Concept**
A social smile is a developmental milestone in infants, typically observed between 2-4 months of age. It is a sign of cognitive and emotional development, indicating the infant's ability to recognize and respond to social interactions.

**Why the Correct Answer is Right**
The emergence of a social smile is a significant milestone in infant development, reflecting the maturation of the infant's brain and nervous system. Social smiling involves the activation of the facial nerve (cranial nerve VII) and the release of neurotransmitters such as dopamine, which play a crucial role in regulating emotional responses. As the infant develops, they learn to associate smiling with social interactions, such as eye contact and vocalizations, which are essential for building relationships and communication skills.

**Why Each Wrong Option is Incorrect**
**Option B:** Sitting without support is a gross motor skill typically achieved between 6-7 months of age, which is beyond the developmental stage of a 3-month-old baby.

**Option C:** Transferring objects from one hand to another is a fine motor skill that develops later, around 5-6 months of age, and requires more advanced hand-eye coordination and dexterity.

**Option D:** Changing position from prone to supine is a gross motor skill that is typically achieved around 4-6 months of age, which is also beyond the developmental stage of a 3-month-old baby.

**Clinical Pearl / High-Yield Fact**
Remember that the emergence of a social smile is a crucial indicator of normal cognitive and emotional development in infants. Any delay or absence of social smiling should prompt further evaluation to rule out underlying developmental or neurological disorders.

**✓ Correct Answer: A. Social smile**