Keep the given modal may and choose the correctly formed formal warning beginning with "Under no circumstances".
Under no circumstances may visitors open the cabinet.✓Richtige Antwort
Under no circumstances visitors may open the cabinet.
Under no circumstances do visitors may open the cabinet.
Under no circumstances may open visitors the cabinet.
Erklärung
The fronted restrictive phrase requires the modal may to come before the subject. Adding do is unnecessary with a modal, while statement order misses the required formal inversion.