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.
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 https://brainly.com/question/8123358.