Answer:
In Arthur Conan Doyle's "The Adventure of the Speckled Band," Holmes asks Helen Stoner to stay in her room for the night because he believes that there is a real and imminent danger to her life. He believes that an intruder may attempt to harm her during the night, and he wants to ensure that she is safe.
Step-by-step explanation:
In the story, Helen Stoner consults Holmes about some mysterious events that have been occurring in her family's estate, including the death of her sister and strange noises at night. Holmes investigates the matter and eventually discovers that the family's stepfather, Dr. Roylott, is attempting to murder Helen in order to gain control of her inheritance.
When Holmes realizes the truth, he advises Helen to stay in her room for the night with the windows open, while he and Watson keep watch from outside. Holmes believes that this will prevent Dr. Roylott from carrying out his plan, as he does not want to be caught in the act. By asking Helen to stay in her room, Holmes is trying to protect her from harm and ensure that she survives until the morning, when he can confront Dr. Roylott and bring him to justice.