After a week, my cousin had not _____ able to think of any good slogans.

• A. be
• B. been
• C. being
• D. become

Answer: (B)

The key idea is expressing ability in the past with a perfect construction. After a week marks a past moment, and the sentence talks about what was not possible up to that point: my cousin had not been able to think of any good slogans. Been is the past participle of be, used with the perfect form to show ongoing ability (or lack of it) up to that past moment. Using be would be ungrammatical after had not, and being is also incorrect because the -ing form doesn’t fit this structure. Become would imply a change of state, which isn’t the intended sense here. So the sentence means that up to that week, he hadn’t managed to think of any good slogans.