**Core Concept**
The question assesses the infant's developmental milestones, specifically their cognitive and motor skills. It is essential to understand the sequence of normal development in infants, including visual tracking, motor control, and social interaction.

**Why the Correct Answer is Right**
The described behaviors suggest that the infant is around 2 months old. At this stage, infants typically can:

* Follow objects to midline, but not past it
* Lift their heads from a flat position, but not yet to a 45-degree angle
* Smile spontaneously and respond to familiar faces, such as their parents
* Track objects, including faces, but may not yet be able to track their own hands

The ability to regard their own hand and follow objects past midline typically develops later, around 4-6 months.

**Why Each Wrong Option is Incorrect**
**Option A:** This age is too early for the described behaviors, as infants at 1 month typically do not have the same level of motor control or cognitive development.
**Option B:** This age is too late, as infants at 3-4 months are expected to have more advanced motor skills, including the ability to lift their heads to a 45-degree angle and follow objects past midline.
**Option C:** This age is too late, as infants at 4-6 months are expected to have more advanced cognitive development, including the ability to regard their own hands and follow objects past midline.
**Option D:** This option is not a valid choice.

**Clinical Pearl / High-Yield Fact**
It is essential to remember the sequence of normal developmental milestones in infants, as delays or abnormalities in these milestones can indicate potential developmental or neurological issues.

**Correct Answer:** C. 3 months