Question: What is the most common object class of image ID 56?
SQL: 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