To solve this problem, we need to determine the antonym of the underlined word "guileless" in the given sentence: "She had a ruddy, open face and an oddly guileless manner."
The word "guileless" means straightforward, sincere, or innocent, without deceit.
- cadaverous: This means resembling a corpse in being very pale, thin, or bony. It does not have the opposite meaning to "guileless." However, since this option is marked as the "correct answer" given, we acknowledge it but note the confusion in the context.
- cacophonous: This refers to a harsh, discordant mixture of sounds. This word is related to sound, not to personality, and is not the opposite of "guileless."
- nauseous: This means feeling like vomiting or experiencing disgust. Like cacophonous, it's unrelated to a person's manner or nature, hence not an antonym for "guileless."
- bumptious: This means self-assertive or proud to an irritating degree. It is related to personality traits and behaviors. It contrasts with the sincerity and modesty implied by "guileless," making it the best antonym among the options provided.
Thus, the best opposite meaning to "guileless" based on the definition is bumptious. It's important to verify the problem statement or options, as "cadaverous" does not perfectly fit as an antonym.