If you're reviewing (AmE) / revising (BrE) for an exam, and the material is provided by the same authorities, for gosh sakes go with (a). It's clearly the best answer.
If you find the grammar in (a) to be a little strange, please let us know so we can discuss it.
(It's absolutely correct, by the way.)
If, on the other hand, you're offended as I am by the s