The correct option is B unwillingly
"Reluctantly" means unwilling or disinclined to do something. Thus, option B is correct.
"Slowly" means at a slow speed. Speed can be slow due to any number of reasons, not only because of reluctance. Thus, this is incorrect.
"Inefficiently" is incorrect as to be reluctant does not mean one is not working at maximum capacity.
"Unhesitatingly" is an antonym of the given word.
Thus, option B is the correct answer.