Final answer:
The correct possessive adjective to fill in the blank is 'ma'. It is used to indicate possession by a female speaker.
Step-by-step explanation:
The correct possessive adjective to fill in the blank is ma.
The sentence should be: J'ai cette brosse à dents. C'est ma brosse à dents.
The possessive adjective ma is used to indicate possession by a female speaker.
Learn more about Possessive adjectives in French