The official accused of espionage was found to be disingenuous.

Disingenuous means not candid or sincere; pretending one knows less about something than one really does. The antonym should convey honesty and sincerity. Scrupulous means very concerned to avoid doing wrong, careful and thorough, honest. Duplicitous, mendacious, and perfidious are all synonyms of disingenuous (meaning deceitful, lying, and treacherous respectively). Therefore, scrupulous is the only antonym in the options.