Choice "A" is correct. Review of assistants' work is necessary to determine whether it was performed in a satisfactory manner and to determine whether the work supports the conclusions presented in the auditor's report.
Choice "d" is incorrect. The responsibility of a firm to establish review procedures (a system of quality control) is distinct from the auditor's responsibility to adequately plan and supervise the work on a particular engagement. Quality control standards relate to the conduct of the firm's audit practice as a whole; GAAS relates to the conduct of individual audits.
Choice "c" is incorrect. Specific audit procedures are performed based on the auditor's judgment and would not necessarily be approved in the professional standards.
Choice "b" is incorrect. Review of an assistant's work would not necessarily be indicative of that assistant's technical training and proficiency. The review process is to determine whether the audit is being performed in a satisfactory manner.