Which event in the story is a key detail in Holmes’s solution of the case?
a. the observation that Mr. Wilson belonged to the Freemasons
b. the walk around Saxe-Coburg Square
c. the invitation to the police agent from Scotland Yard
d. the violin concert at St. James’ Hall
The event in the story that is a key detail in Holmes’s solution of the case is "a. the observation that Mr. Wilson belonged to the Freemasons" since this was the society that had raised suspicions before.
