Final answer:
The account included in a post-closing trial balance is common stock because it is a permanent account that carries over its balance, whereas dividends and expenses are temporary accounts, closed out after the period ends.
Step-by-step explanation:
The account that will be included in a post-closing trial balance is common stock. The post-closing trial balance includes only balance sheet accounts which have a balance after the closing entries have been made. Accounts like dividends, salaries expense, and office supplies expense are temporary accounts that would have been closed to the retained earnings account. Therefore, they do not appear on the post-closing trial balance. In contrast, common stock is a permanent account and reflects the equity of the company's shareholders which remains on the balance sheet from period to period.