In a organisms are particularly susceptible to certain kinds of stimuli in their environments, but the absence of those stimuli does not always produce irreversible consequence

The time period being referred to is the sensitive period. This time period is also referred to as the critical period. The importance of a critical period is such that it must involve certain stimuli from which an organisms learns and acquires traits. If some of these stimuli are not present in the critical period, the organism has difficulty acquiring the trait or skill, and may even find it to be impossible to learn the skill.

The correct answer is sensitive period.
This period refers to a stage in an organism's growth when the stimuli found in the surrounding world can affect the organism in a more dramatic way. Also, this organism is aware of the world around it and will learn many new skills more quickly during this period.