The correct answer is B. Family members set places at the table for relatives who cannot attend.
The original, passive sentence was - Places are set at the table by family members for relatives who cannot attend. In order to revise this sample sentence into active voice, we need to put the subject of the sentence first - those who perform the action of setting the table - family members.
Therefore, options C and D are incorrect because the subject doesn't come first here - these two examples are still passive sentences. Options A and B are active; however, A is incorrect because members who aren't present cannot possibly set the table. Thus, the only correct answer is B.
