Final answer:
The correct partitive to fill in the blank is 'du'.
Step-by-step explanation:
The correct partitive to fill in the blank would be du. In the dialogue, the phrase 'le cahier de Monsieur Lebrun' indicates ownership, and in French, the partitive 'du' is used before a masculine singular noun beginning with a consonant sound. 'Cahier' is a masculine singular noun, and it starts with a consonant sound, so 'du' is the appropriate choice.
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