The correct answer is C. dubious.
A casual appearance would be one that is not unexpected or unusual. A surprising appearance would be one that is unexpected and causes surprise. A certain appearance would be one that is known or predictable. A dubious appearance is one that is questionable or doubtful.
In the case of the stranger in the cottage, the fact that all doors were closed suggests that the stranger should not have been able to enter. This makes the stranger’s appearance questionable or doubtful.
The other options are incorrect because they do not accurately describe the stranger’s appearance. A casual appearance would not be surprising or doubtful. A surprising appearance would not be questionable or doubtful. A certain appearance would not be surprising or doubtful.