Where should I post business promotion in trial balance

To begin with, you don't post to a trial balance. A trial balance is just a listing of all your ending account balances. i.e. after all journal entries and posting have been done. So what you're really asking is what accounts should be used to make the entry, not to post to a trial balance.

Do you have a promotions account? It could go in advertising. The problem is that a company can put this where they want it, and so can a textbook, and we don't have your list of accounts to know what is being used. It might be they want it in just miscellaneous expense if there isn't some place better to put it. i.e. there is no one definitive answer. But it's some type of expense. And then the other side is either cash or accounts payable, depending on whether it's been paid or not.