On image no. 20, identify the attribute ID that is composed of the highest number of objects.
image no. 20 refers to IMG_ID = 20; attribute ID refers to ATT_CLASS_ID; highest number of objects refers to max(count(ATT_CLASS_ID))
SELECT ATT_CLASS_ID FROM IMG_OBJ_ATT WHERE IMG_ID = 20 GROUP BY ATT_CLASS_ID ORDER BY COUNT(ATT_CLASS_ID) DESC LIMIT 1