I haven't seen you ...... a week.

Solution (By Examveda Team)

I haven't seen you for a week.
In this context, "for" is the correct preposition to indicate the duration of time during which the speaker hasn't seen the person addressed. It implies that the speaker has not encountered the person in question for the past week. Therefore, "for" is the most appropriate choice to complete the sentence, conveying the duration of time involved.