**Core Concept**
Developmental milestones in infants are crucial for assessing their overall growth and neurological development. The ability to follow an object with their eyes, hold their neck, and sit with support are significant indicators of a baby's physical and cognitive development.

**Why the Correct Answer is Right**
By 6 months of age, babies typically demonstrate advanced physical skills, including the ability to hold their neck while sitting and follow an object with their eyes up to 180 degrees. However, they still require support to maintain a sitting position, as their core muscles and balance have not fully matured. This is a normal part of their development, as they gradually transition from a lying down to a sitting position.

**Why Each Wrong Option is Incorrect**
**Option A:** At 1 month, babies are still in the early stages of physical development and are unable to hold their neck or follow an object with their eyes beyond a limited range.

**Option B:** By 2 months, babies begin to show some improvement in their physical abilities, but they are still not able to sit without support or follow an object with their eyes up to 180 degrees.

**Option C:** While 4 months is a significant developmental milestone, babies at this age are usually able to sit without support and may begin to show more advanced physical skills than described in the question.

**Clinical Pearl / High-Yield Fact**
Remember that developmental milestones can vary significantly among infants, and some may require more time to achieve certain skills. However, a general rule of thumb is that babies typically reach certain milestones at the following ages: holding their head up while in a prone position by 1-2 months, sitting with support by 4-5 months, and sitting without support by 6-7 months.

**✓ Correct Answer: D. 6 months**