Explanation

The clue innumerable fruitless attempts indicates that Allen lacked the crucial qualification to juggle, so a word that means something like manually coordinated will fit the blank. Both lumbering and awkward are nearly the opposite of what you’re looking for, so eliminate choices (B) and (F). Neither sedate, which means calm, nor implicit, which means implied, make sense in the blank, so eliminate choices (A) and (D).

Both dexterous and implicit can mean manually coordinated, so choices (C) and (E) give you appropriate, equivalent sentences.