Step 1: Understanding the Concept:
Infant socio-emotional developmental milestones include fear responses triggered by unfamiliar individuals.
Step 2: Detailed Explanation:
Stranger Anxiety is a normal, universal socio-emotional developmental milestone that emerges in infants between 6 to 12 months of age (peaking around 8--10 months).
It reflects the infant's increasing cognitive capacity to distinguish familiar primary caregivers from unfamiliar faces and represents secure attachment to parents.
Step 3: Final Answer:
Thus, this fear reaction is referred to as Stranger anxiety, matching option (D).