"Tom gave up the brush with reluctance in his face, but alacrity in his heart."
The clue "but" can help you assume that the missing word will have a contrary meaning to "reluctance." Indeed, we are told that Tom made a face to visibly indicate that he did not want to give up the brush. The word "but" is used to express contradiction, so you can guess that "in his heart," Tom felt differently about it.
Among the possible answers, he word which means the opposite of reluctance is alacrity, which is the behavior of someone who is enthusiastically ready or willing to do something.
