Final answer:
Nonsampling risk is best defined as a mistake or failure in the application or analysis of statistical methods, affecting the reliability of sampling data other than natural variation.
Step-by-step explanation:
The term nonsampling risk refers to an issue that affects the reliability of sampling data other than natural variation; it includes a variety of human errors including poor study design, biased sampling methods, inaccurate information provided by study participants, data entry errors, and poor analysis. Therefore, the best definition of nonsampling risk among the options given would be a mistake or failure of the auditor in the application of a statistical method or in the analysis of results. This can involve issues like poor study design, improper application of statistical methods, or errors in interpreting the data.