2 Big Al's picture and Carla's picture are better than / the best Rocco's picture. His painting isn't good! I think it is worse than / the worst.

Решение:

Во второй части предложения говорится, что картина Рокко не очень хорошая, поэтому её сравнивают с самой худшей. Правильный вариант выделен.

Big Al's picture and Carla's picture are better than / the best Rocco's picture. His painting isn't good! I think it is worse than / the worst.