Final answer:
The correct past participle to complete the sentence is 'écrit'.
Step-by-step explanation:
The correct past participle to complete the sentence would be écrit. In French, the past participle is used with the auxiliary verb 'avoir' to form the compound past tense. In this case, 'j'ai' is the conjugated form of 'avoir' and 'écrit' agrees with the subject 'un email' in gender and number, so it becomes 'écrit'.
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