Final answer:
The correct word to fill in the blank is 'plague,' as it best describes the hero being troubled by doubts during the night.
Step-by-step explanation:
The appropriate word to complete the sentence "Although the hero was confident, doubts would sometimes________ him in the middle of the night." is b) Plague. This word means to cause continual trouble or distress to someone. The sentence would thus read, "Although the hero was confident, doubts would sometimes plague him in the middle of the night." The words encourage, inspire, and ignite do not appropriately capture the nuance of being troubled by something, which 'plague' does.