**Core Concept**
The question is testing the developmental milestones of a baby, specifically the age at which they achieve certain gross motor skills. These skills include tracking objects with their eyes (visual tracking), holding their neck in a stable position (neck control), and sitting without support (sitting unassisted).

**Why the Correct Answer is Right**
At around 4-6 months of age, babies typically develop the ability to follow objects with their eyes up to 180 degrees, indicating good visual tracking skills. They also start to hold their neck in a stable position, which is essential for sitting and other gross motor activities. Most babies are able to sit without support by around 6-7 months of age. This milestone is a critical indicator of gross motor development and is often used as a marker for normal development.

**Why Each Wrong Option is Incorrect**
**Option A:** 2-3 months - At this age, babies are still developing basic gross motor skills and are not yet able to follow objects with their eyes or sit without support.
**Option B:** 8-9 months - While babies at this age are likely to be able to sit without support, they may not have developed the necessary visual tracking skills to follow objects at 180 degrees.
**Option C:** 10-12 months - Babies at this age are likely to have well-developed gross motor skills, but the question is asking for the age at which these skills are typically first achieved.

**Clinical Pearl / High-Yield Fact**
Remember that developmental milestones can vary widely among babies, but certain skills are typically achieved within specific age ranges. Keeping track of these milestones can help identify potential developmental delays or concerns.

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