Answer:
Exposition.
Step-by-step explanation:
The exposition of a literary text is the inclusion of any background information, be it about the setting, the character(s), prior plot events, or any historical context that connects the story. In literature, the exposition appears as a form of 'pre-set' for the story or events that are to come along.
In the given excerpt from Arthur Conan Doyle's "A Study in Scarlet", Watson narrated how Holmes lead his regular life daily. These details constitute the exposition of the story, providing details of the story.
Thus, the correct answer is exposition.