What is the category of the business with medium review length and highest review stars within business ID from 6 t0 9?
category refers to category_name; highest review stars refers to max(review_stars); business ID from 6 to 9 refers to business_id between 6 and 9
SELECT T4.category_name FROM Reviews AS T1 INNER JOIN Business AS T2 ON T1.business_id = T2.business_id INNER JOIN Business_Categories AS T3 ON T2.business_id = T3.business_id INNER JOIN Categories AS T4 ON T3.category_id = T4.category_id WHERE T1.review_length LIKE 'Medium' AND T2.business_id BETWEEN 6 AND 9 ORDER BY T1.review_stars DESC LIMIT 1