## Core Concept
The question tests the knowledge of **gross motor developmental milestones** in infants, specifically the age at which a child can sit without support. Gross motor skills involve the coordination of large muscle groups to perform movements.

## Why the Correct Answer is Right
Sitting without support is a significant milestone that typically occurs around **6 months of age**. At this stage, infants have developed sufficient neck and back muscle strength to maintain an upright position. This milestone is a part of the Denver Developmental Screening Test and is used to assess the normal development of infants.

## Why Each Wrong Option is Incorrect
* **Option A:** Sitting with support usually starts around 4-5 months, but sitting without support requires more strength and balance, which is not typically achieved this early.
* **Option B:** While some infants might start showing signs of sitting without support around 5 months, it is generally considered that most infants achieve this milestone a bit later.
* **Option D:** By 9 months, most infants can not only sit without support but also have started to move towards crawling, standing, or even walking with support. Sitting without support is a skill that usually precedes these milestones.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **developmental milestones serve as guidelines**, and there can be a range of normal development. Factors such as prematurity can affect the timing of these milestones. Clinicians often use these milestones to identify potential developmental delays.

## Correct Answer: C. 6 months.