Based on the fact that Arianna's fears of being fat, her abnormally low body weight, and her fears of gaining weight, her eating disorder is called Anorexia Nervosa.
<h3>What is anorexia nervosa?</h3>
This is an eating disorder that makes a person have an unfounded and irrational fear of being fat even when they are not.
As a result, they try to maintain a low body weight which is abnormal in order to stay healthy. This is what Arianna has.
In conclusion, Arianna has anorexia nervosa.
Find out more on anorexia nervosa at brainly.com/question/8123358.