Answer :
The sentence that states a theme of "The Red-Headed League" and belongs in a summary of the story is:
"When readers learn Spaulding is really a thief, it develops the idea that people may not be who they appear to be."
This sentence is a key theme in the story as it highlights the element of deception and appearances being misleading. The revelation about Spaulding not being who he initially seemed to be underscores the theme of hidden identities and the importance of being cautious about trusting appearances. It adds a layer of mystery and suspense to the plot, emphasizing the idea that things are not always as they seem, and individuals may have hidden motives or secrets.