**Core Concept**
The development of rolling over in infants is a significant milestone in gross motor skills, involving the coordinated effort of multiple muscle groups and neural pathways. This process is influenced by the maturation of the nervous system, particularly the corticospinal tract, and the strengthening of neck and trunk muscles.

**Why the Correct Answer is Right**
Rolling over is a complex motion that requires the integration of sensory input from the visual, vestibular, and proprioceptive systems. As the infant gains strength and coordination, they begin to lift their head while on their stomach, bringing their shoulder and hip closer to the ground. This motion is facilitated by the development of the rectus abdominis and external oblique muscles, which enable the infant to rotate their trunk and bring their arm and leg towards the midline. The correct answer is the age range during which this milestone is typically achieved.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as rolling over is not typically achieved at this age. Infants usually begin to roll over between 4-6 months of age.
* **Option B:** This option is incorrect as it is not a specific age range. Rolling over is a developmental milestone that occurs within a specific age range.
* **Option C:** This option is incorrect as it is too late for the typical age range of rolling over. Infants usually begin to roll over before they start crawling.

**Clinical Pearl / High-Yield Fact**
Infants typically begin to roll over between 4-6 months of age, and this milestone is often accompanied by the development of other gross motor skills, such as lifting their head while on their stomach and bringing their arm and leg towards the midline.

**Correct Answer: D. 4-6 months.**