**Core Concept**
The gross motor milestone of sitting without support is a significant developmental achievement in infants, marking a transition from prone to upright posture. This milestone is an indicator of the maturation of the neuromuscular system, particularly the development of trunk muscles and the integration of sensory information.

**Why the Correct Answer is Right**
Sitting without support is typically achieved around 15 months of age, as the infant's trunk muscles, including the latissimus dorsi, trapezius, and rhomboids, become stronger and more coordinated. The development of the vestibular system and the integration of visual, proprioceptive, and vestibular inputs also play crucial roles in this milestone. As the infant gains confidence and balance, they can maintain an upright posture without support.

**Why Each Wrong Option is Incorrect**
**Option A:** Sitting without support is not typically achieved before 12 months, as the infant's trunk muscles and overall neuromuscular system are not yet mature enough to support this posture.
**Option B:** While some infants may sit without support as early as 12 months, this is not a typical age for achieving this milestone.
**Option D:** Sitting without support is not a milestone typically achieved at 24 months, as most infants are able to sit independently by 12-18 months.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that developmental milestones can vary significantly among infants, and some may need additional support or therapy to achieve these milestones. A thorough developmental assessment by a pediatrician or a qualified healthcare professional can help identify any potential concerns.

✓ Correct Answer: C. 15 months