**Core Concept**
Macular differentiation refers to the development and maturation of the macula, a critical region in the retina responsible for central vision and fine detail. This process is crucial for visual acuity and is closely linked to fetal development. The macula's anatomical and functional development is a complex process involving multiple cellular layers and neural connections.

**Why the Correct Answer is Right**
The correct answer is related to the timeline of fetal development, particularly the formation and maturation of the retina. Macular differentiation is a key milestone in this process, occurring relatively late in fetal development. Understanding the timeline of retinal development is essential for appreciating the complexities of visual system formation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it suggests an age that is too early for macular differentiation.
**Option B:** Similarly, this option is also incorrect as it does not align with the known timeline of retinal development.
**Option C:** This option is incorrect because it proposes an age that is not supported by the established stages of fetal development related to the visual system.
**Option D:** This option is also incorrect as the correct age for macular differentiation is earlier than this option suggests.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the retina, including the macula, continues to develop after birth, with significant maturation occurring in the first few years of life. This has implications for visual development and the assessment of visual acuity in infants and young children.

**Correct Answer:** D. 4-5 months