## Core Concept
The question assesses the developmental milestones of a child, specifically focusing on gross motor skills and social interaction. It tests the understanding of typical developmental stages in children.

## Why the Correct Answer is Right
At 12 months (or 1 year), children typically achieve several developmental milestones. They can walk while holding one hand, which demonstrates improved balance and gross motor skills. They also start showing social behaviors like releasing an object to another person on request, indicating an understanding of simple commands and social interaction. Playing a simple ball game is another milestone that aligns with this age, reflecting developing hand-eye coordination and social engagement.

## Why Each Wrong Option is Incorrect
**Option A:** 9 months - At this age, children are beginning to walk independently but might still need support. They may not have fully developed the ability to play simple games or consistently respond to requests to release objects.

**Option B:** 15 months - While children at 15 months continue to refine these skills, the described behaviors are generally expected before this age.

**Option D:** 18 months - This age is too advanced for the described milestones, as most children would have surpassed these by then.

## Clinical Pearl / High-Yield Fact
A key point to remember is that while these milestones provide a general guideline, there is a range of normal development. Clinicians often consider 12-18 months for walking independently and notable social and motor skill advancements.

**Correct Answer: C. 12 months.**