Sadid is 2 years old and his older sister, Jenn, is 5 years old. While they're playing one day, Jenn puts a barrette in Sadid's hair. Sadid gets upset and states that he doesn't want to be a girl, and that he likes being a boy. Jenn laughs because she understands that putting the barrette in Sadid's hair did not make him a girl. What has Jenn achieved that Sadid has not

Gender constancy.

Explanation:

Gender constancy: In psychology, the term "gender constancy" is described as a theory that a child tends to develop a specific sense of gender with time & eventually or finally come to comprehend that his or her "biological sex" is being permanent and fixed.

The concept of "gender constancy" was proposed by an American psychologist named Lawrence Kohlberg.

In the question above, Jenn has achieved "gender constancy" that Sadid has not.