Choice "A" is correct. Since a material weakness in internal control is important enough to merit attention by those charged with governance, it would also be considered a significant deficiency.
Choice "b" is incorrect. The auditor is required to separately identify and communicate significant deficiencies and material weaknesses.
Choice "c" is incorrect. Significant deficiencies and material weaknesses are generally communicated to the appropriate parties after the audit is complete. They may, at the auditor's discretion, be communicated during the audit, but there is no requirement for immediate communication.
Choice "d" is incorrect. A material weakness is a significant deficiency that results in a reasonable likelihood that a material misstatement in the financial statements will not be prevented or detected/corrected. Not all significant deficiencies will be material weaknesses.