We don't know how we are to recompense you for the trouble you have taken.

Answer: Option A

Reward : a thing given in recognition of service, effort, or achievement.
Help : make it easier or possible for (someone) to do something by offering them one's services or resources.
Praise : express warm approval or admiration of.
Thank : express gratitude to (someone), especially by saying ‘Thank you’.
Recompense : pay or reward (someone) for effort or work.

Synonym of Recompense is Reward