How can an organization ensure the objectivity and impartiality of the internal audit function?

Comprehensive and Detailed In-Depth Explanation:

According to ISO 19011:2018, Clause 5.1 (Impartiality):

Internal auditors must not audit areas where they have direct responsibilities to avoid conflicts of interest.

Outsourcing (C) is not required, as long as impartiality is maintained internally.

Thus, B is the correct answer.