Choice "C" is correct. The nature and scope of an assurance engagement is determined by the internal auditor, however, the nature and scope of a consulting engagement is subject to agreement with the engagement client.
Choice "b" is incorrect. There is no requirement that the scope of assurance engagements be determined by the external auditor and consulting engagements typically involve an agreement with the engagement client.
Choice "d" is incorrect. Objectivity is required for both assurance and consulting activities.
Choice "a" is incorrect. Independence requirements (e.g., the one year restriction on internal audit activities covering areas in which the auditor had direct involvement) are more restrictive for assurance activities than consulting activities.