Choice "A" is correct. In evaluating the reasonableness of an estimate, an auditor would normally concentrate on key factors and assumptions that are (1) significant to the accounting estimate, (2) sensitive to variations, (3) deviations from historical patterns, or (4) subjective and susceptible to misstatements and bias.
Choice "b" is incorrect. The auditor need not concentrate on key factors or assumptions that are consistent with those of prior periods, since estimates based on such assumptions are less likely to be misstated.
Choice "d" is incorrect. The auditor need not concentrate on key factors or assumptions that are similar to industry guidelines, since estimates based on such assumptions are less likely to be misstated.
Choice "c" is incorrect. The auditor need not focus on factors that are objective and not susceptible to bias, since estimates based on such assumptions are less likely to be misstated.