Choice "A" is correct. The auditor's direct personal knowledge (obtained through observation, examination, inspection, or recalculation) is one of the most reliable forms of evidence.
Choice "b" is incorrect. Scanning of trial balances may indicate areas where more attention should be focused (e.g., unusual balances, zero balances, etc.), but would seldom provide reliable evidence in and of itself.
Choice "d" is incorrect. Inquiry of client personnel provides evidence that is not particularly reliable, which is why it often needs to be corroborated by the auditor.
Choice "c" is incorrect. A comparison of beginning and ending retained earnings may provide information about certain transactions and events (e.g., dividends, income, etc.), but would not in and of itself provide evidence supporting those items.