Final answer:
The appropriate form of the adjective "blanc" is "blanc".
Step-by-step explanation:
The appropriate form of the adjective "blanc" to fill in the blank in the sentence "En Martinique, le sable est" is "blanc". In French, adjectives agree in gender and number with the noun they modify. Since "sable" (sand) is singular and masculine, the correct form of the adjective is also singular and masculine, which is "blanc".
Learn more about Adjectives in French