Humans learn languages best in their childhood. In fact, some research indicates that if a child doesn't learn to speak a formal language before the age of 6, they may never learn to speak well at all. This would best illustrate which of the following? a. Critical period
b. Normative age graded changes
c. Individual differences
d. Cultural specificity

Option A: CRITICAL PERIOD

Explanation:

Humans must be exposed to language during a specific maturational state in order for language to be full acquired

human development deals with the study of age-related changes in  human behavior, way of  thinking, their emotion, and personality.

The way children learns is quite different from that of adult.

critcal period covers  the basic idea that there may be essential/important  periods in development when humans  is especially sensitive to the presence or absence of some particular kind of experience or influence. The Critical period in language acquisition is that period of time that humans must be exposed to language during a specific maturation state so as that language to be fully learned.

Humans must be exposed to language during a specific maturational state in order for language to be full acquired

Example is during the 6 month of a child's growth,  children begins babbling and becomes variable and language specific by 12 months and when this babling does not occur during this period, it becomes difficult for that child to fully acquire this basic skill.