Final answer:
The most appropriate preposition to fill in the blank is 'de'. It indicates possession or belonging.
Step-by-step explanation:
The most appropriate preposition to fill in the blank is de.
So the correct sentence is: 'No es mi camisa roja. Es la camisa roja de Juan.'
This preposition indicates possession or belonging. It is used to show that the red shirt belongs to Juan.
Learn more about prepositions in Spanish