Final answer:
The correct possessive adjective to complete the sentence is 'mes'.
Step-by-step explanation:
In the sentence "Ce sont mes cd", the correct possessive adjective to complete the sentence is mes. Possessive adjectives in French agree in gender and number with the noun they modify. In this case, the noun "cd" is plural, so we use the plural form of the possessive adjective, which is "mes" for both masculine and feminine nouns. Examples: "Ce sont sont livres" (These are my books) or "Ce sont sacs a dos" (These are her backpacks).
Learn more about Possessive adjectives in French