The correct option is D Eager
The word 'reluctant' refers to being unwilling or hesitant.
Option A, doubtful refers to feeling uncertain about something.
Option B, worried refers to feeling anxious or troubled about actual or potential problems.
Option C, anxious refers to feeling worry, nervousness or unease about something with an uncertain outcome.
Option D, eager refers to strongly wanting to do or have something.
Thus, according to the meaning, the word 'eager' is the opposite of the word 'reluctant'.