Choice "D" is correct. Reviewing purchase contracts and other legal documents might aid the auditor in identifying omitted liabilities (for example, an obligation to make a certain purchase, or a document evidencing a lawsuit).
Choice "a" is incorrect. Inquiry of senior support staff and recently departed employees is unlikely to detect a liability omission by management, since management probably would not have allowed this information to become well-known among employees.
Choice "b" is incorrect. If a liability is mistakenly omitted from recording, it will not be in the financial statements. Therefore, such an omission is unlikely to be detected by checking the mathematical accuracy of the financial statements.
Choice "c" is incorrect. While reviewing the articles of incorporation and corporate bylaws provides good background information about a company, these procedures would be unlikely to provide evidence regarding omission of a liability in the current year's financial statements.