**Core Concept**
Neck holding, also known as neck retraction or torticollis, is a common postural abnormality in infants, characterized by the head being tilted to one side and the chin being drawn towards the shoulder. This phenomenon is primarily due to the immature development of the cervical spine and the resultant imbalance of the neck muscles.

**Why the Correct Answer is Right**
Neck holding typically begins around 2-3 months of age, as the infant's cervical spine starts to develop and the neck muscles become more prominent. The exact age may vary, but the majority of infants exhibit this behavior between 2-6 months of age. The underlying mechanism is thought to be related to the imbalance of the sternocleidomastoid and trapezius muscles, which are responsible for neck movement and posture.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it is too early for neck holding to occur, as the cervical spine is still developing and the neck muscles are not yet prominent.
**Option B:** Incorrect because it is too late for neck holding to be a common phenomenon, as the infant's neck muscles have matured and the cervical spine is fully developed.
**Option C:** Incorrect because it does not specify a particular age range, making it too vague and not a reliable answer.

**Clinical Pearl / High-Yield Fact**
It is essential for pediatricians and parents to recognize neck holding as a normal developmental phase, rather than a cause for concern. However, if the infant's neck holding persists beyond 6 months of age or is accompanied by other concerning symptoms, it is crucial to consult a pediatrician to rule out any underlying conditions.

**Correct Answer: D. 6 months**