The correct answer is: External auditors must be satisfied that the work of internal audit is of adequate quality and that internal audit work is adequately documented and planned; The internal auditors must be properly trained, and their work supervised and reviewed.

ISA 610 Using the work of internal auditors requires that external auditors must be satisfied that the work of internal audit is of adequate quality and adequately documented and planned if they wish to rely on it. It also requires that external auditors are satisfied that internal auditors are properly trained, and that their work is supervised and reviewed.

Internal auditors are sometimes seconded to work for the external auditors, in which case they should follow the instructions of the external auditors, however this is not necessary in order for external auditors to rely on their work.

The responsibility for the external audit report is always the responsibility of the external auditor, regardless of the extent to which internal audit work has been used. Internal auditors do not take any responsibility for the external audit report. It is not necessary for internal audit work on material areas to be performed by qualified accountants in order for external auditors to rely on it.