**Core Concept**
Morris (or Moro) reflex is a reflex that appears in infants and is characterized by a distinctive response to a sudden loss of support or a loud noise. It is a normal reflex in infants up to a certain age, but its persistence beyond that age can be an indicator of abnormal neurological development.

**Why the Correct Answer is Right**
The Moro reflex typically disappears by 4-6 months of age as the infant develops more control over their muscles and the nervous system matures. Persistence of the reflex beyond this age can indicate developmental delay or neurological disorders such as cerebral palsy. The reflex is mediated by the vestibular system and involves the stimulation of the proprioceptive receptors in the neck and trunk muscles.

**Why Each Wrong Option is Incorrect**
**Option A:** The Moro reflex usually disappears by 4-6 months, so it is normal to see it in infants up to 3 months of age.
**Option B:** While the Moro reflex typically disappears by 4-6 months, it is still possible to see it in some infants up to the 4th month, making this option incorrect.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the disappearance of the Moro reflex by 6 months of age is a significant milestone in infant development, and its persistence beyond this age should prompt further evaluation for potential neurological disorders.

**✓ Correct Answer: D. 6th month**