ingenuous â‰ ingenious
The GRE loves to use words that people tend to mistake for other words. Ingenuous means candid, open, unrestrained, naive. A good synonym is artless (another confusing word — artless means lacking artifice, not lacking works of art).
Ingenious, of course, means characterized by cleverness or ingenuity. A person is a genius; his or her work is ingenious.
The quack made a career out of bilking vulnerable seniors by setting up tables at the mall and performing fake medical tests, thus convincing people to pay for his “cures.” You could say he was ingeniously disingenuous.