Which part in the following sentence is incorrect?They would not have able to plan the details of the trip without your help.

Explanation

The given statement indicated the use of present perfect tense due to the use of the auxiliary verb 'have'. Since the verb, 'able' is not in the past tense hence, 'been' should have been used after able. This is why the phrase, 'have able to' is grammatically wrong in the sentence. The rest of the sentence is grammatically correct hence Option B is the correct option.