She had a ruddy, open face and an oddly guileless manner.

Ruddy means having a healthy red color (typically of a person's face). The antonym should describe an unhealthy, pale appearance. Cadaverous means resembling a corpse—pale, gaunt, and thin, which is the direct opposite of ruddy. Cacophonous refers to harsh sound; nauseous means feeling sick; bumptious means self-assertive in an arrogant way. Therefore, cadaverous is the correct antonym.