Choose the right answer for concersion of the following sentence as directed. He is very dull and he can not understand his lesson (Make it simple)

He is very dull that he can not understand his lesson.

He is too dull to understand his lesson.

He is too dull that that he does not understand his lesson.

He is so dull that he does not understand his lesson.

He is being dull that he can not understand his lesson.