I'll see you again in a week. The answer is B. What's the difference between the two?

From the translation point of view, B means see you in a week, and D means see you in a week.

Grammatically speaking, the complete form of I'll should be I will, which means the future.

according to

In+ time period: used with future tense

After+ time period: used with past tense

After+ time point: can be used with future tense.

Because a week is a time period, which means both the time period and the future tense, only in is used.

To sum up, choose B in.