Final answer:
Sera thinks Mr. Thorne addressed Mr. Rostanov as 'father' and 'papa' to indicate a familiar and affectionate relationship.
Step-by-step explanation:
Sera thinks that Mr. Thorne addressed Mr. Rostanov as 'father' and 'papa' because it indicates a familiar and affectionate relationship between them.
These terms of endearment suggest that Mr. Thorne sees Mr. Rostanov as a paternal figure, someone he looks up to and respects.