Choice "D" is correct. A client-imposed scope limitation indicates that the client might be hiding errors or irregularities that could result in a material misstatement of the financial statements.
Choice "c" is incorrect. Rescheduling staff in response to acceptance of a new audit engagement is a normal activity for CPA firms and does not impact audit risk.
Choice "a" is incorrect. The hiring of a specialist generally decreases risk.
Choice "b" is incorrect. The fact that the client's financial reporting system has been in place for 10 years might be indicative that system's errors have been resolved and that the system has integrity.