Which of the following parties is accountable for ensuring adequate support for conclusions and opinions...

The chief audit executive (CAE) is accountable for ensuring that adequate support exists for the conclusions and opinions formed by the internal audit activity. When relying on the work of external auditors, the CAE must ensure that the work is sufficient and appropriate to support the internal audit conclusions. This involves reviewing the external auditors' work to confirm its relevance and reliability, and integrating it into the internal audit processes as necessary.

The Institute of Internal Auditors (IIA) Standard 2050 - Coordination and Reliance.

IIA Standard 2340 - Engagement Supervision