some of the sentence are grammatically correct and some are incorrect. Find which part of the sentence has an error and mark that part. If there is no error,mark that part as your answer
`("Unemployment has reached")/a` `("a level that would have been not")/b` `("thinkable two years ago")/c` `("No error")/d`

Explanation

(b).have not been. in place of .have been not.