Some of it is discretionary, but you can appeal to the Ombudsman.
Either way, I highly recommend telling the whole truth. If the officer asks you "have you ever been arrested?" you should say "yes, but the charges were dropped and my record was expunged."
Be prepared to show paperwork.
Prepare yourself for the possibility of an on-the-spot denial by the officer interviewing you, though you can appeal later.
*Note: I am not an attorney and the above does not constitute legal advice.