What is the most common object class of image ID 56?
the most common object class of image ID 56 refers to MAX(COUNT(OBJ_CLASS_ID)) where IMG_ID = 56;
SELECT T2.OBJ_CLASS FROM IMG_OBJ AS T1 INNER JOIN OBJ_CLASSES AS T2 ON T1.OBJ_CLASS_ID = T2.OBJ_CLASS_ID WHERE T1.IMG_ID = 56 GROUP BY T2.OBJ_CLASS ORDER BY COUNT(T2.OBJ_CLASS_ID) DESC LIMIT 1