If you have any further questions, do not hesitate to call me is a professional way to close a refusal message. a. true b. false

In my opinion, the correct answer is b. False. This kind of sentence invites for a further discussion, and would be appropriate in a job application letter, cover letter, or something like that. However, a refusal letter should explain the reasons for refusing something that are not debatable. It is basically the end of written correspondence, so there is no reason to contact anyone further. If you leave more room for contact, it would either mean that you aren't sure about your decision, or that you are expecting a different kind of offer (should that be the case, this is not a professional way to do it).