The statement that is a theme is: "Encouragement can be a source of strength."

Explanation:

• A theme is a central idea or underlying message in a story.
• The other statements describe plot events or descriptions :
  • "Two friends travel in search of the dreaded monster." — plot summary
  • "The cedar forest is a marvel of towering trees." — setting description
  • "Humbaba is an evil monster who is defeated." — plot detail

Only the statement about encouragement conveys a broader, universal message or lesson, which makes it a theme.