Marla is interviewing the primary suspect of an investigation.

Marla cannot conclude that the suspect is being deceptive based solely on foot tapping. The ACFE materials warn that no single behavior proves anything. Behavioral observations must be evaluated against the individual’s baseline and interpreted in clusters, not in isolation. Restless behavior might indicate nervousness, discomfort, cultural habits, stress, physical conditions, or other innocent explanations. Although such behavior might be considered during an interview, it cannot safely support a conclusion of deception by itself. Option B is too absolute because restlessness can sometimes be considered a possible behavioral cue. Options C and D are incorrect because they overstate the reliability of nonverbal behavior. Therefore, the proper conclusion is that Marla should not isolate the behavior to infer dishonesty.