Step 1: Understanding the Concept:
Developmental psychology differentiates between rigid biological critical periods and flexible sensitive developmental windows.
Step 2: Detailed Explanation:
In human developmental biology:
A Sensitive Period is a biologically determined time window in which an individual's brain development is exceptionally receptive to specific environmental experiences (such as language acquisition or social attachment).
If the appropriate environmental stimuli are absent during this period, development can still occur later, but it is much more difficult and less optimal.
Step 3: Final Answer:
Thus, this optimal time span is called a Sensitive period, corresponding to option (A).