**Core Concept**
At 9 months of age, infants typically demonstrate significant cognitive, motor, and linguistic advancements, marking important milestones in their development. These milestones are often used as indicators of a child's overall health, development, and potential developmental delays.

**Why the Correct Answer is Right**
One of the key developmental milestones attained at 9 months of age is the ability to sit without support. This skill is a significant indicator of gross motor development, reflecting improvements in balance, posture, and overall motor control. The development of this skill is closely linked to the maturation of the cerebellum and the integration of sensory information from the visual, vestibular, and proprioceptive systems.

**Why Each Wrong Option is Incorrect**
**Option A:** Standing without support is typically achieved at around 12 months of age, making it a less likely correct answer.
**Option B:** Crawling is a skill that is usually mastered between 7-10 months of age, making it an early developmental milestone.
**Option C:** Walking independently is usually achieved at around 12-14 months of age, making it a later developmental milestone.

**Clinical Pearl / High-Yield Fact**
Infants typically achieve the skill of sitting without support before standing without support, and this sequence is often used as a clinical indicator of normal development.

**Correct Answer: D. Sitting without support.**