ExplanationBecause you know that the puzzle in question is one of the more difficult puzzles, you would expect Sandra to be, well, “puzzled” by it. Enraged and incensed are an intriguing pair, but you have no reason to believe Sandra was made angry by the puzzles. Smitten can’t work here because the correct idiom is “smitten with.”
Impressed could work, but it doesn’t have a pair to match with. Confounded and flummoxed are the best choices.
Hence options Confounded, flummoxed are best fit!
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