Final answer:
The essential factor in evaluating the sufficiency of evidence is that it must be reliable and relevant.
Step-by-step explanation:
The essential factor in evaluating the sufficiency of evidence is that it must be reliable and relevant. This means that the evidence should come from credible sources and be directly related to the topic or issue being investigated. Evidence obtained from only internal sources or based on management's assertions may not always be reliable or objective, so these options are not considered essential factors.