Experts recommend that invigilation must continue: a. for at least 7 days. b. for at least 21 days. c. for an indefinite period determined by the individual investigator. d. for at least 14 days.

The answer is D. for at least 14 days

Explanation:

In most investigations of a fraud suspect investigations should begin by checking personal and company records, interviewing co-workers, performing invigilations.

If there are symptoms or red flags that a fraud may be occuring, Experts recommended that invigilations must continue for at least 14 days so as to be sure there are no signs or occurrence of fraud