Choice "A" is correct. Unrealized gains on available-for-sale securities should properly be recorded in other comprehensive income. If such gains were erroneously recorded in the income account for trading securities, this might be discovered through comparison of the current year and prior year revenues and expenses (assuming the error occurred only in the current year, and not in the prior year).
Choice "b" is incorrect. If payroll taxes were properly accrued and recorded, there is unlikely to be a significant change in revenues and expenses for the current year as compared to the prior year. Payables would not be part of the comparison of revenues and expenses.
Choice "d" is incorrect. In times of worsening economic conditions, one would expect the annual provision for uncollectible accounts to increase. Since this answer option indicates that the provision was inadequate, it would appear that the client did not increase the provision appropriately. Investigating changes in revenues and expenses would not be likely to identify this error, since failing to increase the provision would likely result in there being little change between the two years.
Choice "c" is incorrect. An increase in property tax rates should cause a corresponding increase in accrued property tax expense; however, the question indicates that the appropriate increase was not recorded in the current year. Investigating changes in revenues and expenses would not be likely to identify this error, since failing to increase the expense would likely result in there being little change between the two years.