Final answer:
The correct preposition to fill in the blank is "sur" which means "on" in English.
Step-by-step explanation:
The correct preposition to fill in the blank is sur.
"Le sac est sur le bureau" means "The bag is on the desk". This preposition is used to indicate location or position, specifically on top of or covering something.
For example, if you have a book on a table, you would say "Le livre est sur la table".
Learn more about Prepositions in French