When is it MOST important for an IS auditor to apply the concept of materiality...

The concept of materiality is most important for an IS auditor to apply when planning an audit engagement, because it helps the auditor to determine the scope, objectives, procedures and resources of the audit. Materiality is the degree to which an omission or misstatement of information could affect the users’ decisions or the achievement of the audit objectives. By applying the concept of materiality, the auditor can focus on the most significant and relevant areas of the audit and avoid wasting time and effort on trivial or immaterial matters. The other options are not as important as planning an audit engagement, because they are either based on or affected by the materiality assessment done during the planning phase. References:

ISACA, CISA Review Manual, 27th Edition, chapter 1, section 1.31

ISACA, IT Audit and Assurance Standards, Guidelines and Tools and Techniques forIS Audit and Assurance Professionals, section 12022