Choice "A" is correct. The auditor is generally required to follow the guidance provided by the Standards, and should be able to justify any departures.
Choice "c" is incorrect. On rare occasions, the auditor may depart from the guidance provided by the SASs, but he or she must justify such departures.
Choice "d" is incorrect. Lack of familiarity with a SAS is not a valid reason for departing from its guidance. The auditor is expected to have sufficient knowledge of the SASs to identify those that are applicable to a given audit engagement.
Choice "b" is incorrect. The cost associated with following the guidance provided by a SAS is not an acceptable reason for departing from its guidance.