**Core Concept**
Sitting without support is a significant milestone in a child's physical development, marking the transition from a prone to a sitting posture. This skill is typically acquired during the first year of life, as the infant develops the necessary strength and coordination in their muscles, particularly in the back, abdominal, and leg regions.

**Why the Correct Answer is Right**
The ability to sit without support is achieved when the infant's muscles have developed the strength to maintain a stable posture against gravity. This is made possible by the maturation of the muscles in the back, particularly the erector spinae, which helps to straighten and stabilize the spine. Additionally, the development of abdominal muscles, such as the rectus abdominis, also plays a crucial role in maintaining a stable posture. As the infant's balance and coordination improve, they are able to sit without support, typically around 6-7 months of age.

**Why Each Wrong Option is Incorrect**
**Option A:** 8 months - This age is slightly later than the typical age for sitting without support.
**Option B:** 4 months - At this age, most infants are still working on developing the necessary strength and coordination to sit without support.
**Option C:** 9 months - This age is also later than the typical age for sitting without support.

**Clinical Pearl / High-Yield Fact**
Remember that sitting without support is a key milestone in a child's physical development, and its absence can be an indicator of motor skill delay or other underlying conditions.

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