Which word could best replace the word vigilant without changing the overall meaning of the passage

In order to answer this correctly, you must know the meaning of vigilant (keeping careful watch for impending danger) as well as the meanings of all the choices given. Which two words are similar in meaning? When you have that answer, take out the word vigilant, and put the new word in its place. Does it still make sense? If so, you most likely have the correct answer.