Which of the following specifications in an internal audit charter is the most important factor...

The most important factor in the internal audit activity’s independence specified in an internal audit charter is the description of the internal audit activity's reporting structure. This element defines to whom the chief audit executive reports functionally and administratively, which is critical to ensuring the independence and objectivity of the internal audit function from management and other potential sources of bias.

The IIA's International Standards for the Professional Practice of Internal Auditing on independence and objectivity.