The abstract should start with a sentence that contains the essentials of the original document, avoiding repeating the words of the title. It should only include significant points, be clear and concise, avoiding peripheral comments and generalities. ... Time and verb must not be dissociated within the summary.
Since a participle takes the form: verb + -ing, the only word in this sentence which has such a form is B, 'challenging'. 'Puzzle' is a noun, 'crossword' is an adjective, 'created' is a verb.
