**Core Concept**
The question tests the understanding of **developmental milestones** in infants, specifically the age at which certain physical and cognitive abilities are typically achieved. This includes **visual tracking**, **neck control**, and **sitting abilities**.

**Why the Correct Answer is Right**
A baby who can follow an object with 180°, hold their neck, and sit with support but not without support is demonstrating skills expected around **6 months** of age. At this stage, infants have developed sufficient **cervical spine stability** and **upper body strength** to hold their neck up and support their upper body while sitting with assistance.

**Why Each Wrong Option is Incorrect**
**Option A:** 1 month is too early for these developmental milestones. **Option B:** 3 months is also early, as babies typically cannot sit with support at this age. **Option C:** 5 months is close but still a bit early for consistent sitting with support. **Option E:** 9 months is too late, as babies should be able to sit without support by this age.

**Clinical Pearl / High-Yield Fact**
Remembering that **sitting with support** usually occurs around **6 months** and **sitting without support** around **7 months** can help in assessing an infant's developmental progress.

**Correct Answer:** D. 6 months