Answer:C. The visitor left his stick behind.
Step-by-step explanation:
Your question is referring to the excerpt from ''The Hound of the Baskervilles'' by Sir Arthur Conan Doyle in which Sherlock is describing his visitor at the beginning of the first chapter.
''Mr. Sherlock Holmes, who was usually very late in the mornings, save upon those not infrequent occasions when he stayed up all night, was seated at the breakfast table. I stood upon the hearth-rug and picked up the stick which our visitor had left behind him the night before.''
After that, Sherlock picks up the stick and read what is written on it and then he is asking Watson about his opinion on it.
He was considered as the absent-minded because he left something with his name and evidence that is showing that he was there. If he was aware, he would not do that. In that way, Sherlock got all information about him.