**Core Concept**
The question assesses the developmental milestone of a baby's ability to sit without support, which is a significant indicator of their physical and neurological maturation. This milestone is closely related to the development of the musculoskeletal and nervous systems.

**Why the Correct Answer is Right**
The ability to sit without support typically develops between 6 to 8 months of age. This is because the baby's neck muscles, back muscles, and abdominal muscles have developed sufficiently to support their body weight. Additionally, the nervous system has matured to the point where the baby can coordinate and maintain the necessary muscle contractions to sit upright without support. This developmental milestone is a critical indicator of the baby's overall health and neurological development.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because babies typically develop the ability to sit without support between 6 to 8 months of age, not before 4 months.
**Option B:** This option is incorrect because while some babies may start to show interest in sitting without support around 5 months, they are not typically able to do so independently until around 6-8 months.
**Option C:** This option is incorrect because babies are usually able to sit without support by 7-8 months of age, not after 9 months.

**Clinical Pearl / High-Yield Fact**
A baby's ability to sit without support is a critical milestone in their development, and any significant delay in achieving this milestone should prompt further evaluation by a pediatrician to rule out any underlying developmental or neurological disorders.

**Correct Answer: C. 7-8 months.**