Final answer:
The adverb 'unexpectedly' best completes the sentence, indicating that the flying fish's leap was a surprise to the observers.
Step-by-step explanation:
The correct word to fill in the blank in the sentence 'A flying fish leaped out of the water ___, startling all of us.' is unexpectedly. This adverb fits in the context because it describes something happening without warning or preparation, which is in line with the examples provided that depict sudden and surprising actions in different scenarios.