Final answer:
The correct preposition to fill in the blank is 'de', indicating possession or origin.
Step-by-step explanation:
The correct preposition to fill in the blank in the sentence is de. So the sentence should read: "Esos libros son de la profesora Martín."
The preposition de is used to indicate possession or origin. In this case, it shows that the books belong to the teacher Martín.
Example: "El coche de María." (Maria's car).
Learn more about Prepositions in Spanish