While normally calm, 10-year-old gretta becomes very shy and highly distressed when she is in an unfamiliar setting with people she does not know or faced with new challenges. gretta would best be categorized as

behaviorally inhibited.

Explanation:

Behavior Inhibition refers to a temperament or personality style where social anxiety develops when an individual faced a new or strange situation. Children showing behavior inhibition tends to feel distress and withdrawn when exposed to unfamiliar people or the environment. These children tend to ignore the new people or situations and stay vigilant of their surroundings.

As per the question, Gretta become shy and distressed when exposed to new unfamiliar setting and new challenges, it is a classic example of behavioral inhibition.