**Core Concept**
The question is testing the development milestone of hand dominance in a child, specifically the transition from bilateral to unilateral hand use. This is an important indicator of neurological development and cognitive function.

**Why the Correct Answer is Right**
The transition from bilateral to unilateral hand use is a significant milestone in child development, typically occurring around 6-7 months of age. This is because the child's brain is maturing and developing the ability to control and coordinate hand movements. At this stage, the child begins to favor one hand over the other, indicating the development of hand dominance. This is a normal and essential part of child development, laying the foundation for future fine motor skills and cognitive abilities.

**Why Each Wrong Option is Incorrect**
**Option A:** 2 months - This is too early for the transition from bilateral to unilateral hand use, as the child's brain is still in the process of developing and maturing.
**Option B:** 9 months - While some children may exhibit unilateral hand use by 9 months, the typical age range for this milestone is between 6-7 months.
**Option C:** 12 months - By 12 months, the child's hand dominance is typically well-established, and they may be using one hand preferentially for most activities.

**Clinical Pearl / High-Yield Fact**
Remember that the transition from bilateral to unilateral hand use is a normal part of child development, and it does not necessarily indicate any underlying neurological or cognitive issues. However, if a child is significantly delayed in achieving this milestone, it may be worth consulting with a pediatrician or child development specialist.

**Correct Answer: C. 6-7 months**